I don't fancy three in a bed. He's wearing my knickers. I'm scared of flies. My boyfriend keeps prodding me with his fork... Being on the receiving end of 1,000 complaints every week – and finding the energy to answer them – requires patience and compassion beyond most of us. So what drives Deidre Sanders, agony aunt extraordinaire, to keep solving the nation's personal predicaments after 30 years in the job?

You think you've got problems? About 1,000 times a week, every week, for the past 30 years, total strangers have been writing to Deidre Sanders and asking her to resolve their impossible situations. This one keeps sleeping with his aunt. That one worries her breasts are too small. A third is letting an obsession with the television chef Gary Rhodes destroy his marriage. All are crying out for help. And Deidre replies to every single one.

So, all right. They and you might have legitimate claims to the occasional long dark night of the soul. Logistically, though, Deidre's worries – unless you run the Chinese civil service or the NHS – make your life look, frankly, like a cakewalk.

Luckily, Deidre, "Dear Deidre", is not just the country's best-known and longest-serving agony aunt, about to complete a third decade at The Sun: Dear Deidre is also a well-oiled and stoutly sensible advice-giving machine. And Dear Deidre has a system. Without it, the whole enterprise – quite unlike anything attempted by any other practitioner – would fall apart. "When I talk to my colleague agony aunts about the job we're doing – I appreciate their abilities, but the scale of what we do here is just different," she says proudly. "But we have had 29-and-a-half years to perfect it."

On a recent Tuesday, I visited the nondescript converted flat – about an hour from London, and, metaphorically at least, still further from the diktats of Rupert Murdoch – where that system is housed. Deidre is at her computer, putting the finishing touches to next Saturday's column. In an adjoining room, a crack team of letter-writing lieuten-aunts, armed with cups of tea and a bank of good sense, sit typing suggestions and juggling problems, the air-traffic controllers of the office's cluttered emotional skies. A tight-knit group – all of whom have been with Deidre for years, some from the very beginning – they are fond of group fact-finding exercises, like a memorable trip to a Walthamstow condom factory. The best part? The flavours room.

Nearly every available flat surface in the warren-like space is covered in a ream of pea-green print-outs, each sheet of which represents a problem that is at once tiny and enormous. "It's like a sea of paper," Deidre observes. "I've had to take two carrier bags full down to the car just to get them out of the way." Those problems are placed in piles according to urgency, and those piles are stamped with a name; every name signifies that one of the in-house agony aunts will be taking that particular slice of worry in hand, and doing her best to make it a little easier to bear.

Not all of those worries are as glamorous as the soap operatics that appear in the newspaper. Yvonne, who has worked with Deidre for 29 years, keeps a roughly alphabetised index of the subject areas that readers write in about. She opens it at random. "HIV," she begins. "Hair problems, hepatitis, homework, holidays, hysterectomy." The aunts erupt in laughter. Truly, all human life is here.

Next door, Deidre is dealing with a rather different sort of difficulty. The sub-editors have called, her assistant Jeannie (another 30-year stalwart) explains; there's a problem with the Saturday lead, which, it is felt in Wapping, might be a little rich for The Sun readers' breakfast tables. Deidre casts a judicious eye over the offending copy. "A stain on my girlfriend's dress looked like Baileys," she reads aloud, "But it sure didn't smell like it." She puffs out her lower lip. "I thought it was funny," she says, and shrugs. "Never mind."

One thing is clear: 64-year-old Auntie Deidre is no prude. You wouldn't know it from the mugshot that accompanies her column, though, in which the woman with a predilection for references to presidential semen looks like a slightly doddery old sweetheart. The effect is presumably deliberate: she has been portrayed as cosily as possible to emphasise that this is the woman you can tell anything. She does not look like a woman with an informed and detailed opinion on how webcam sex is destroying the sex lives of married couples. She looks almost unworldly.

In reality, she is anything but. No one could subject themselves to the nation's emotional crises for three decades and not come out of it with some street smarts, and her eyes, as well as being friendly, are unmistakably sharp. When The New Review's photographer sets up a moderately complex picture, Deidre is nothing but amenable, but it's also clear there are other uses of her time she would consider more productive. "She would have risen quite high in the Army," says Phillip Hodson, a fellow of the British Association for Counselling and Psychotherapy and former advice columnist himself, who has known Deidre for 30 years. "You can see Deidre in Helmand province. There's always a doyenne. It was Marje. Now it's her."

"Marje" is, of course, the late Ms Proops, who casts a considerable shadow over her successors, even in death. For many modern readers, the advice column may as well have begun in 1948, when Proops reluctantly took on the auntly mantle at the Daily Herald when her predecessor died on the job: it's her brisk, down-to-earth style that makes itself felt most strongly in her successors' work, not least Deidre's.

In fact, the advice column goes back a good deal further. He might not have been happy with the term, but the world's first agony uncle, according to Robin Kent's 1979 history of the profession, Aunt Agony Advises (now sadly out of print, and from which all of the following examples are taken), was one John Dunton.

Dunton was a 32-year-old printer who hit on the idea of answering readers' personal questions when he founded a periodical called The Athenian Gazette in 1691, and to the modern sensibility, not all of his advice sounds quite right. One early and fretful correspondent asked, "Which is the greatest sin, to be a Night-Walker [or prostitute], or to rebel against one's parents?" These days, we call that sort of thing sex slavery, and get in touch with the police, but Uncle John took a different view. "Either of 'em big enough [a sin] for Damnation," a distinctly unhelpful Dunton snarled. "But to return a direct answer, we refer you to the order of their setting down in the Ten Commandments, where duty to parents is pressed before adultery is forbid."

Since those enlightened days, the form has gone through numerous iterations, all of which hewed closely to the morals of the times. When a young romantic asked The Englishwoman's Domestic Magazine in 1856 how, "now that Gretna Green is done away with", she might get married "secretly and expeditiously", the regretful answer came, with barely repressed excitement: "It is not our business to teach persons how to evade the laws. We could imagine circumstances where we might, in confidence, impart the coveted knowledge, but we would rather not be asked."

And so it continued. Even in the first half of the 20th century, most agony aunts remained po-faced. Only with Proops did things begin to change, and even she was swimming against the tide at first. The difference was bracing. "What would you do with a husband like mine?" she was asked in an early column. "He won't take me out, never makes so much as a cuppa, refuses to help me wash up and doesn't care that I haven't had a new dress in the two years we've been married."

Proops, audibly outraged, came back with a zinger. "Your old man is a right selfish so-and-so," she fumed. "If I could resist bashing him over the head with the teapot, I'd leave the washing up in the sink, and clear off with a girl friend for a weekly beano. I'd also rifle the housekeeping."

Deidre's approach is a little less abrupt, but there is no doubt that she found her feet in the age of Marje. When The Sun poached Deidre from the Star in 1979, she was still very much a junior figure. "It was a much smaller operation at the Star," she says. "They definitely had my raw version." For her family – solid Yorkshire stock – her job was not always easy to take. "I had an aunty who said, 'Ee Deidre, why couldn't you get a proper job?'" she remembers. "That was when I had a guide out called 'A Hundred and One Questions About Sex'. It was in all the newsagents."

At first, when Sanders graduated from Sheffield University in 1966, her ambitions had indeed been limited to a more conventional journalistic path: she had edited the student paper, and while the home-made publication she produced as a small child (for a putative readership of cats) did include an advice column, that was only one part of an appealing mix. "First and foremost she considers herself a journalist," says John Runeckles, a friend since she was a 20-year-old student. "She fell into writing an advice column, really." Her early career took her from Nova to Woman's Own via The Sunday Times, but as the years went by, her direction began to crystallise. Her journalism would always be driven by an urge to be helpful, to expose problems and put them right.

"I've always been interested in family dynamics," she says. "Why do some people work through their problems and others not? What affects children? A friend of mine complained that I always liked to lift the bonnet up in a relationship and see what made it tick." Indeed, her own family – she has been married for 40 years, and has two grown-up daughters – occasionally note the toll her professional interest can take.

"It's them that comment most on that," she says, ruefully. "This job does make you see the potential for disaster in situations. If you tell me you've just met this wonderful girl, I do start thinking – well, this, that and the other could happen here. I always see the problems coming up." Not that her professional status holds much sway with her offspring. "I'm sure my daughters don't listen to me any more than anybody else's daughters listen to them. They do when they want to, and when they don't want to, they don't."

The fascination with puzzling out a relationship has, Deidre surmises, a great deal to do with her own past. Raised in Harrow, Middlesex, where her parents were both teachers – her father the headmaster of a secondary modern – her childhood was an apparently stable affair. In fact, though, her mother was an alcoholic. "I think [my career choice] was to do with that experience," she says. "Of being at home as a child with someone who was very unhappy, and very self-destructive, and it all being kept very hush-hush. That was definitely a big motivating factor in me later feeling very attracted to a job which is about opening up problems, about giving permission to people to admit we all have them."

To the casual reader of the column, some of this may sound less than convincing. Answers in the paper are sometimes as short as 40 words, usually below a much longer description of the problem, which generally benefits from some ripe details. "We had sex – four or five times – on the sofa," writes one confused youth with an unhealthy interest in his aunt. "It was magic." "Tell your aunt the fun must stop here," Deidre replies; her answers, although invariably stuffed with sound advice, can occasionally border on the peremptory, and it always jars a little if readers are directed to a premium-rate advice line for more information. Nor do the notorious photo stories – in which glamour models mull over their deepest traumas dressed mostly in their underwear – do much to alleviate such doubts.

But according to Philip Hodson, Deidre has never been a great fan of that part of her work. "I think she's always found the picture stories the most difficult part of being an agony aunt," he says. "But she has transcended that, and she's no puritan. She appreciates that the purpose is to increase circulation – that it's there as entertainment. What countless editors who have sent her countless memos saying they want more sex in the column have failed to appreciate is that it's a department of the social services as well."

In fact, however racy the problems might get, the fundamental wisdom of Deidre's response is rarely in doubt. And, crucially, every letter that gets polished up and published also receives a much longer, and much less brisk, private response. Those letters, ' mostly written by her staff and given the OK by Deidre, are not for public consumption, but for a sense of the tone, consider the astonishing library of leaflets that the office has prepared for common problems, from B07L – "Hate being hairy?" – to V07L – "Sex games and you".

"I'm very glad you got in touch," they begin, before going on in exactly the solicitous, warm voice that anyone obsessed with Gary Rhodes at the expense of his fiancée will probably need. ("Kick the habit of watching Gary Rhodes," was the pithy conclusion of the published version, in a sentence that represented probably the most indisputable advice that an agony aunt has ever given.)

That voice is a model of grounded generosity, always finding ways to remind the writer that they are not alone, and never losing the lightness of touch that reminds you that almost nothing is irrevocable, that troubles are all part of life's rich tapestry. "If I were you, I wouldn't go over the rights and wrongs of it too much," she coos, in R27L, "Mend a broken heart". "I still receive regular complaints from women whose men think foreplay can consist of a quick grope and then bingo," she snorts, in leaflet S30L, surely the most succinctly titled of the set: "Fed Up With Wham Bam?"

To read these leaflets, and then consider them fanning out across the country, accompanied by a personal missive that climbs into the pit of loneliness alongside whichever poor soul has written in, lends credence to the argument that Bernard Levin put forward about Marje Proops in a 1976 review of a compilation of her letters. "Does she do any good?" Levin asked. "I cannot see how anybody... can be in any doubt that she does an enormous amount, possibly – of direct, practical good, at any rate – more than any other single person in the country." Likewise, the leaflets makes a mockery of the classic song by the Proclaimers, "Dear Deidre", which whines: "Dear Deidre can you tell me/ Where I'm going wrong/ I'm following your advice/ But my wife's still gone/ She left me for my girlfriend/ The four-faced cows/ We had three in a bed/ But there's only two now."

In fact, though, her advice was there for the taking. Had the band only got in touch in the first instance, they would have received a copy of leaflet V06L, "Fancy three in a bed?" Deidre's cautionary question is almost prescient. "If one of you is keen to have someone else of the same sex in your bed," she asks, "are you perhaps more gay than you've perhaps admitted, or even realised?"

As proofs of expertise go, it's a convincing one. But even if she rates herself higher than the Proclaimers do, Deidre makes less grandiose claims for her own work than Levin did for Proops'. Perhaps it's a constitutional necessity for agony aunts: as plainly as she is a believer in the value of what she does, Deidre's self-assessment is a fraction equivocal. "I'm not always confident about it," she says, "and in particular about whether I've managed to work out what the underlying problem is, and whether people are likely to actually act on what I've suggested. Lots do, and lots don't."

The concrete results that the day job denies can at least be found in her extracurricular efforts to further democratise counselling, and as part of the Social Work Task Force. In the first of those roles, things have gone well: she rallied her fellow agony aunts to Whitehall in December last year, and came away with £60m for counselling services in schools.

"She's been at the centre of that whole project," says Ed Balls, the secretary of state for children, who still sounds slightly gobsmacked. "We got on really well. She's very knowledgeable and brave and she's a great campaigner and communicator. [In the meetings] it's impossible to get a word in edgeways, to be honest." And now, Deidre smiles, with a self-effacement that you don't entirely buy, "To our amazement, we have regular meetings with secretaries of state!"

The Social Work Task Force has been less of a success, largely because of many in the profession's opposition to her position on the panel in light of The Sun's fervid and often unreasoned critique of social workers during the Baby P case. For Deidre, who is plainly constitutionally disposed to sympathise with the social worker's lot, this can't be easy. How does she reconcile that viewpoint with the publication she works with? "Well, I agree that they have had a really tough time," she begins, seeming uncomfortable for the only time in our conversation. She points out some moves the paper has since made that she considers positive, and then pauses, and starts again: "Of course there are differences. But if I edited The Sun and had only my views in there, it would not be the wide-ranging successful paper it is. They're very good at editing the paper and they trust me to be good at editing the problem page."

She has a point, of course. If she finds herself on the same side of the river as The Independent and The Guardian, there are no useful bridges to be built. It is a job that requires a balancing act like no other. If, as PG Wodehouse once said, the novelist must either "make a sort of musical comedy without music and ignore real life altogether" or "go right down deep into life and not give a damn", it is surely only the agony aunt – and Deidre, with her audience of entertainment- hungry millions, most of all – who is expected to do both. And there can be little doubt, on the other hand, of the value of her river-crossing work at The Sun, in which Deidre's page remains – after the sport section – the most popular part of the paper.

Still, she must, sometimes, get sick of it. "Well," she allows, "I certainly do intend to retire at some point." Not for her, Marje Proops' exit feet-first. To the question what she would do next, she has a stock answer: she'd like to write crime thrillers. "I have this fantasy about having this agony-aunt sleuth," she laughs. To the rest of us, it rather seems as if that's what she is already. n

'Is my man affected by the Moon?'

9 November 2002

Dear Deidre, do you know if people can be affected by the Moon? Because my man definitely seems to be. I am at my wits' end with my partner of four years. We are in our thirties and he is honest, hard-working, loyal and loving – except at certain times of the month. Then he undergoes a personality change, getting moody and verbally abusive. I've noted the pattern of these outbursts and they occur during the phases of the Moon, with the worst times during the full Moon. Without help our relationship is doomed.

Deidre says: I've never seen proof that people's moods can be affected by the phases of the Moon, but couples can certainly affect one another. Your menstrual cycles may alter your mood so you react differently, either triggering his irritability or making you less tolerant of his foibles. Whatever the causes, you must talk it over with him. Ask what he thinks makes the difference and how you can both work together for more even harmony. If you get stuck, arrange to see a Relate/marriage guidance counsellor.

13 November 2002

Dear Deidre, I have been scared of flies since I saw the horror film 'The Fly' and it is ruining my life. I am a 24-year-old woman. Recently I saw the biggest, scariest and ugliest fly I have ever encountered. I ran into the kitchen but my family just laughed at me. How can I overcome my fears?

Deidre says: You may have seen The Fly when something in your life was making you anxious and vulnerable. If you can work out what that was and deal with those real fears, you will be able to separate them from your irrational fear of flies. My advice line today explains self-help for fears and phobias.

20 December 2003

Dear Deidre, I am fed up with my partner's childish behaviour. He will prod me with his fork when we're eating. He'll throw water over my hair when I have just blow-dried it. He is always trying to wrestle with me. I can cope up to a point but he never notices when I have had enough. I'm becoming wary of him touching me in case it turns nasty. He just tells me to lighten up.

Deidre says: If his parents treated him like this, perhaps he only knows how to express tender feelings in a jokey way. But his behaviour has become a form of bullying. Read Power Games by Kay Douglas and Kim McGregor.

5 December 2005

Dear Deidre, my boyfriend likes to wear my underwear when we make love. I used to find it a turn-on but now I'm put off. We're both 22 and have been together for a year. We've always enjoyed an adventurous sex life and never been afraid to experiment. One day I came home to find him wearing my undies. He wanted to make love and it was really passionate. Now it seems he needs to wear my lacy knickers to get him in the mood for sex. This is changing the way I feel about him.

Deidre says: If it's not you that's turning him on, it's natural you're feeling hurt. Tell him how this is affecting you and ask what his priorities are. He may have issues he needs to sort out. My advice line on cross-dressing will help.

Problems selected by Aisha Christison

Why do we write to agony aunts?

By Virginia Ironside, The Independent's veteran advice-giver

Before I became an agony aunt I often wondered what it was that prompted someone with a problem to put pen to paper and write off to a complete stranger for help. Who were these desperate saddos who had no one else to turn to?

The answer, I discovered, after spending years as an agony aunt, was interesting. The truth is that lots of people are far too shy to confide their problems in anyone face to face. They fear they might burst into tears, or that the person they were confiding in might turn around and condemn them. They feel they might not be given the time to tell everything or, worse, they fear that the person they're talking to might laugh in their face. And anyway, who could they confide in, and trust, completely? If it were a friend, might that friend not, one day, tell other people about the problem, or suddenly remind them of it at a vulnerable moment?

Some people find that talking to a stranger on the phone is comforting. They ring the Samaritans. Others talk to their doctors – but often their doctors don't have the time. And some write to agony aunts.

They write to agony aunts because they feel they can trust them. Agony aunts are total strangers, yet, by reading their (usually) kindly and sensible advice over the months, readers do trust them. If their advice was mocking or jokey, they probably wouldn't risk writing in. But usually readers can rely on getting a compassionate answer. They also probably trust anyone who "has their picture in the paper". It's odd, but I'm occasionally recognised in the street by complete strangers, and they always greet me warmly, as if they know me. They forget that although they know me, in a kind of a way, I don't know them.

Writing in itself is often also very therapeutic. I can't count the number of letters I've received that have ended with the words, "Reading this through, I know just what you are going to say" or "Now I've written all this down, I think I know what I must do..." It clarifies the thoughts. Whether the agony aunt replies or not is often not the point. It is in the identifying of the problem that the answer, paradoxically, often lies.

I used to think that the arrival of the internet, with all its information and self-help, would replace agony aunts and make us redundant. But I now realise that there are moments when all of us need to have a way of getting our thoughts in order and, hopefully, getting back a little bit of understanding and kindness from a non-judgmental stranger.