I meet Rosemary Conley - author of the perpetually best-selling Hip and Thigh Diet - at a hotel in central London. She is already up in the suite when I arrive, all big lacquered hair and gleaming lipstick and super-tight little body perched primly and thinly on the edge of the sofa.

Of course, I do not work my way towards her in any way which would give her a glimpse of my backside. What fool would? Instead, I kind of edge around the wall, while firmly keeping my back to it. Are you feeling well, Rosemary asks. I am quite well, thank you, I reply. It's just that I don't want you to view my bottom, you being an expert on bottoms and doubtless knowing a nasty one when you see it. "But," she cries, "you have nothing to worry about! You're so slim!" Naturally, I'd intended to get quite tough on Rosemary. Naturally, I had planned to come over very feminist and scold her for pushing the slimming ideal, something imposed on women by oppressive males in the interest of maintaining the patriarchy and being able to go to the football on Saturday afternoons and not mow the lawn. But blow all that rubbish now. I'm slim! I'm slim! I've got nothing to worry about! Rosemary Conley herself says so! And she, of course, should know.

Rosemary's hip and thigh book has dominated the best-seller lists for more than nine years. All her subsequent books - Rosemary Conley's Complete Hip and Thigh Diet, The Hip and Thigh Diet Cookbook, The Inch Loss Plan, The Metabolism Booster Diet and so on and so forth - have proved instant best-sellers, as have the videos. In fact, she is here today to launch her latest video, Rosemary Conley's Ultimate Fat Burner (pounds 12.99). This features Rosemary doing a lot of terribly frightening, bouncy, stretchy things in a multi-coloured, floral leotard. It is wholly draining and exhausting, and would doubtless have been even more so had I bothered to get out my chair. As it was, I felt quite faint after 10 minutes.

As far as I can work out, Rosemary releases a book or a video - or both - at least once a year. As such, I wonder if she'll shortly run out of body parts to focus on, and be reduced to doing The Total Eyelid Diet, say. She doesn't think so. "Certainly, with the fitness videos, you can't run out of ideas because there is always new music, new routines, new formats. But, yes, I do sometimes get diet-fatigued."

In fact, now I think about it, The Total Eyelid Diet is not such a bad idea. It could involve winking a lot, picking up strange men on the Tube and having so much sex that you forget to eat. What do you think, Rosemary? "It's, um, quite an interesting thought," she says. You're not going to nick it, are you, I ask worriedly. "No. Certainly not," she retorts indignantly. "I've never stolen anything in my life." Of course, Rosemary Conley has a brilliant sense of humour and loves my excellent jokes. She just seems to be having a hard time showing it today.

Anyway, she is immensely rich. Aside from the books and videos, and her weekly spots on This Morning and the bi-monthly Rosemary Conley Diet and Fitness Magazine, there are also the Rosemary Conley Diet and Fitness Clubs. Launched in 1993, this is now the fastest-growing franchise in the UK, with more than 2,300 classes held weekly for 70,000 members. Rosemary drives a Bentley (number plate ROS 1E.) She owns a big house in Leicestershire with horses and acres of land. Her fingers sparkle with diamonds. How much is she worth exactly?

"I wouldn't know," she says. Why on earth not? "I'm just not interested." Oh, come off it. "I'm not. Money means absolutely nothing to me." But the Bentley? The house? The diamonds? "I am very happy about having these things. I appreciate what I've got. I'm not saying I don't. But what I am saying is that I'm not one of those people who wants to count up exactly how much I'm worth."

Rosemary Conley is a committed Christian. Christians, while allowed to diet a lot and have nice houses and motors and horses and sing like Cliff Richard, should not be overly interested in money for money's sake, so Rosemary isn't. Rosemary is not, I suspect, a woman who leaves much to chance. Her mind can't ever surprise her, because she's given it to God. Her body can't ever surprise her because she watches it too closely. She always has to be in control, in other words.

No, the photographer cannot take pictures as we talk, because he might get a profile, and she won't allow profiles because she hates her nose. "Too big. Too hooked." When did she last let herself go? "On the eating front?" she asks. On any front, I say. "Well, at one point, I started going back to eating butter on bread rolls in restaurants." You wild and reckless thing, you. "Yes, it's so easy to slip back. But I stopped doing it one Lent a couple of years ago and I haven't done it since."

Last night, she went out for dinner and yes, had a pudding - "a souffle thing". But that's OK, because "I didn't have cream with it" and she made up for it this morning by having half her usual breakfast. Which is? "One and a half ounces of All-Bran instead of three ounces." Miss Conley is 50. It seems to me that there is something quite sad about a 50-year-old woman measuring out All-Bran in a hotel room after a night out, but I don't doubt that it makes her happy. Best leave nothing to chance.

People who are obsessed with their own weight tend to be so, most psychologists would agree, because it gives them something to control in a world which, they feel, is mostly beyond their control. Is this true of Rosemary? Perhaps.

Her childhood was, for the most part, quite straightforward. She was born in Leicestershire to Celia, a housewife with a mercantile streak - she invented a nylon cap to go over hair rollers which she sold through Harrods - and Oswald, a hosiery wholesaler. She was always very close to her father.

"He was an ever so kind, good, hardworking man," she says. "When I was eight, his business went bust because other people went bust on him. He went into voluntary liquidation but then took out a bank loan to pay off all his creditors, which he did, even though it took him 10 years. He was incredibly decent.

"My mother died 18 years ago but my father is still alive. Sadly, he is in the advanced stages of Alzheimer's now. It started six or seven years ago when he started pouring milk into the sugar bowl. I spoke to him every day until six months ago when there stopped being any point, as he didn't know who I was. My stepmother, Mabel, looks after him for four weeks out of six. For the other two weeks, he's in respite care. Last time I went to visit him in respite, there was a lady there who was 57 - 57! Just seven years older than me. And she was completely out of it!"

Does Rosemary mind getting older, then? "Put it this way. I'd like things to stay where they are now." No, she has never and will never consider plastic surgery. "I don't believe in interfering with nature. That's why I've never had my nose done." But aren't you interfering with nature by going about and telling women they shouldn't have hips and thighs? I mean, we're meant to be pear shaped, aren't we? Yes, she agrees, we are. But why be a Comice when you can be Conference? "And you feel so much better about yourself."

She left school at 14 for a secretarial course; she met Phil, her first husband and an accountant, when she was 16. They were married when she was 21 and had a daughter, Dawn. It was a good marriage all told, she thinks. Trouble was, she got fat.

She had been a skinny child, and never much interested in food. But here she was, a wife with housekeeping money and her own kitchen and a subscription to one of those cordon bleu part works. "I made coq au vin! Meringues. Cakes! And ate them!" she recalls with considerable horror. Her weight went up to 10st 3lb - not huge, even for her 5ft 2in. But it felt monstrous to her. "I hated my weight gain. Hated it. My husband didn't mind, but I cried all the time."

For some years, she fought a losing battle against fat, taking off a stone then putting it on again through bingeing on five litres of ice cream at a sitting, or lumps of cheese spread with butter. Eventually, she took herself off to Weight Watchers, which helped bring her weight down - but didn't do much for her hips and thighs - and led to her starting her own diet class in her kitchen. Sagg - slimming and good grooming - eventually grew into a chain which she sold to IPC for pounds 50,000. The money allowed her to part amicably from Phil. "We'd just drifted apart."

Not long afterwards, she became unwell, suffering from gallstones. She ended up in hospital - an event which, I reckon, is the pivotal moment in the Rosemary Conley life story. Here she was, ill, with no husband, no business, and those obstinate hips and thighs. Yes, she did feel very out of control and she would, she thinks, have had a nervous breakdown if two things hadn't happened. Firstly, she chanced upon a Christian book, The Power of Living, which converted her and made God appear before her in a dream, saying "I shall provide". Then, her doctors told her if she wanted to avoid surgery she would have to go on a low-fat diet and take more exercise. She lost a lot of weight and, miraculously, a lot of it went from her thighs and hips. She then wrote about her experiences - "to help other women".

Of course, there had been diet books before. And there have been umpteen since. And prescribing a low-fat diet coupled with exercise isn't exactly revolutionary. So what made and continues to make The Hip and Thigh Diet so enduringly popular? First off, I reckon, it's the name. Mention hips and thighs and you quite literally go right to the bottom of the average British woman's neurosis. Then there's the fact Rosemary is who she is. She isn't a doctor. She isn't a Jane Fonda. "I'm just an ordinary woman offering sensible advice that works," she says. "People can identify with me."

Yes, she says, she is sure that God approves of a life dedicated to spreading the low-fat word. "We should stop all this nonsense about not being vain and that we shouldn't be dieting," she says. "God didn't make us fat! We made ourselves fat!" Still, there must be a better way of serving God, surely? "I also give talks about my Christian faith," she says. "I give 12 a year."

She now lives with her second husband, Mike Rimmington, a one-time TV engineer who is now her manager. Both are wholly dedicated to God and the Rosemary Conley business, which doesn't appear to leave time for much else, not that Rosemary seems to need any hobbies or anything. Certainly, she doesn't seem to need any emotional outlets. No, she doesn't read. Or listen to music. Or go to the cinema. She only occasionally watches telly. (She likes The Bill and Pie in the Sky, "although that chap could do with going on a diet".)

So what does she do when she's not praying or bouncing about on videos? "I do like flower arranging. My favourite flower? The lily, although you have to watch out for the pollen because it stains." Once a year, she goes on holiday to Austria with slimmers from her clubs. I wonder what going to Austria with a group of slimmers is like. Do you all stand outside the cake shops, looking at the strudels and dribbling all over whatever strasse you happen to find yourself huddled on? "Oh no. We have a super, lovely time. We stay at the same hotel where they make us low-fat food."

`The Total Eyelid Diet', published by Dross Books, will be available from next week, price pounds 498. Steep, I know, but if you're only going to have One Good Idea in your life, you might as well make it pay.