So much for British reserve. Thousands of couples now share their sexual secrets with complete strangers. Clare Rudebeck discovers what's driving us to the couch

" Love is the answer, isn't it?" says Liam Neeson as Dr Alfred Kinsey in the new film about the great sexual enquirer. "But sex raises a lot of very interesting questions." How many minutes should it last? Why can't I orgasm? Why can't I have multiple orgasms? How many times a night should we do it? And should I be thinking about the washing-up? It may be more than 50 years since Kinsey first told us what everyone was doing in bed - in seminal works such as Sexual Behaviour in the Human Male - but the hang-ups his reports revealed are still very much with us.

And where there are such anxieties, there's a market. A thriving industry has evolved to listen to and advise on our sexual hopes, fears and expectations. Pioneered by the American researchers Masters and Johnson in the 1970s, sex therapy is now available to all; Relate, Britain's largest provider, gives almost 50,000 hours of advice on making love every year.

Yet, like the activity it scrutinises, sex therapy can be less than orthodox. You can have surrogate partner therapy (having sex with your therapist); online hypnotherapy (sex tips from a stranger); and sex coaching (for people who think they're already good, but want to be better).

Tomorrow night, the six-part Channel 4 show The Sex Inspectors brings the sex therapists into our homes. The volunteers putting their love lives on screen include a man who lost interest in sex after the birth of his baby, a couple who've done it all and are now bored, and a woman who pestered her husband's libido away. The couples will be shown practising what has been preached, via a video link to their bedrooms.

There are fears that the programme will make us still more obsessed with other people's sex lives. Phillip Hodson, a fellow of the British Association for Counselling and Psychotherapy (BACP), says the show could lead to a "battery of false sex norms".

But, at between £30 and £150 per sex-therapy session, the important question must surely be: how valuable and effective is the advice these experts offer?

Charlotte Ross and Jamie Gold, from Essex, started sex therapy after Charlotte told Jamie she'd been faking her orgasms during their year-long relationship. "Jamie and I had a great sex life," says Charlotte, 30, a full-time mother to her daughter, Amelia, three, from a previous relationship. "But I've never been able to relax in front of a man and have an orgasm. I didn't want to spoil our sex life, so I faked it. I didn't want him to think there was something wrong with me." Then Charlotte told Jamie the truth. "And I wished I hadn't, because our sex life became very strained. He wanted to know why I'd lied to him and I became obsessed with being able to orgasm."

Charlotte and Jamie chose to appear on The Sex Inspectors. The couple were filmed outside and inside the bedroom, and were then advised by the series' two "sexperts", Tracey Cox and Michael Alvear. Charlotte was told to give Jamie a bath before having sex, to help him to relax and slow down in bed. And Charlotte had to practise masturbating with photos of Jamie next to her to help break down her inhibitions.

The breakthrough came when Charlotte admitted that she thought that most women orgasm through penetrative sex. It's actually about 30 per cent. "Once the pressure was off, everything clicked," Charlotte says. "I still haven't had an orgasm through penetrative sex, but I'm now having non-penetrative orgasms whenever I want." Jamie, 29, is also happy and shows no signs of a bruised ego. "It's definitely improved our sex life. It's more relaxed and enjoyable. Neither of us is thinking, 'What's going to happen tonight? Will she or won't she?'"

Tracey Cox, 43, has no formal training in sex therapy. A former editor of Australian Cosmopolitan, she has counselled couples about sex through the media for 20 years, after she specialised in sex therapy in her undergraduate psychology degree.

Therapists at Relate do a two-year training course, theory and practice, in order to qualify. Paula Hall, a relationship and sexual psychotherapist with Relate, says that untrained therapists risk making sexual problems worse. "They may miss deeper psychological issues. Superficially, the inability to orgasm is about letting go, but there may be other issues involved, such as negative experiences in childhood."

Hall often helps women with anorgasmia - the inability to reach orgasm. Like the TV Sex Inspectors, she treats it by giving a couple "homework" to do. "You can talk about a sexual problem until the cows come home," says Hall. "But in order to get over it, you need to practise." This behaviourist approach is common to almost all sex therapy. "You start off with increasing sensory awareness; so, basically, you are going to have a bath. A lot of women who are anorgasmic take showers. They're busy women who don't take much notice of their bodies - it's just that thing that holds their brain."

The pressures of modern life are often to blame for both premature and delayed ejaculation, Hall says. "Men who suffer from delayed ejaculation are usually used to very high stimulation, and to masturbating quickly. So, when it comes to enjoying the pleasures of the flesh, their bodies don't know how to respond."

Hall spends much of her time as a therapist persuading people to slow down. "Once you have got a woman who is anorgasmic to have a bath, you encourage more self-touching. Her partner is going to learn about what feels good for her, but she's got to learn that first. Then you start discovering the erogenous zones - and you haven't even got to the genitals yet."

Hall says her approach has a 95 per cent success rate in her private practice. "And some failures are because the couple split up: the reason they're not having sex is because they don't like each other," she says.

However, Phillip Hodson of the BACP believes the average success rate of sex therapy is much lower. "It's not true that 95 per cent of people will be restored to function through sex therapy. I don't know of any intervention with that success rate."

Charlotte and Jamie sought help quickly. However, most British couples would rather risk their relationship than talk about sex with a stranger, even one who is qualified. Dr David Delvin, a founder of the Institute of Psychosexual Medicine, says: "People often leave it until things are disastrous. Often the marriage is on the brink of breaking up or has broken up, or they are on their second or third marriage."

Becky Morris, 40, and her partner John Norman, 38, have just started sex therapy through the NHS. "The problem is that we have different libidos, mine being low," says Becky, who has been with John for 10 years. "We probably tolerated the situation for so long because when we do have sex - once a month, say - it's very good." Lack of desire is the most common female sexual problem, Relate says; last year, it was cited by half the women attending their sessions.

Some therapists, including Brenda Hill of Topic Counselling Services, believe that television itself may be partly to blame. "We're seeing an increase in problems of desire and libido, and perhaps this is because there's too much sex on TV," she says.

After two sessions, Becky and John are unsure whether the therapy will help. Becky says: "There has been a lot coming out that has been unsaid. Over the years, sex has largely been at my behest, which has given me undue power. John said he thought it might be too late. I suppose it will be make or break."

Couples are often at their lowest ebb by the time they seek sex therapy, but there is currently no framework to protect them from exploitation. Sex therapy, like all psychotherapies, is unregulated. "Anyone can say they are a sex therapist," says Marj Thoburn, who chairs the professional standards board of the Institute of Psychosexual Medicine. "Prostitutes might consider that what they do is a form of sex therapy, and presently no one could challenge that."

Sam Van Rood, 31, was a conservation campaigner in Australia before moving to London four years ago to work as a sex coach. He doesn't claim to be a qualified therapist, but says he's developed his skills through personal experience and testing his theories on friends. "I'm a coach, not a therapist," says Van Rood, who charges £1,500 for an eight-to-10-week course. "I offer men who are good in bed the opportunity to be exceptional. It is like sports training; I take people up to the next level."

Mathias Sudres, 26, a pianist from south London, signed up with Van Rood. "I wasn't happy with my sex life," he says. "I thought, 'That can't be it.'" After 15 sessions, he says he is a greatly improved lover. "It opened the door to my sexuality, and I've learnt how to connect with my partner emotionally."

Van Rood boasts that his coaching has impressive physical results, enabling men to keep an erection for at least 45 minutes. But he stresses that this isn't really the point: "Most men are very goal-orientated with sex. If you compare it to driving along a beautiful coast road, most men would want to drive as fast as possible to their destination. I teach them to slow down, have fun along the way, stop and enjoy the scenery."

In America, the movie Kinsey, which recently opened in New York and Los Angeles, has upset the religious right. One Christian ministry called it "rank propaganda for the sexual revolution and the homosexual agenda". The writer and director, Bill Condon, says the film, which opens in Britain early next year, is "a sort of litmus test for one's own ideas about sexuality".

Paula Hall of Relate, The Sex Inspectors and Sam Van Rood may have rather different approaches, but their message is broadly similar: stop worrying about what everyone else is doing, slow down and learn to enjoy it. If the reaction to Kinsey and The Sex Inspectors tells us anything about our attitude towards sex and sexuality, it's that we're very good at getting wound up about it. And if sex therapy teaches us anything, it's that we're never going to have good sex until we calm down.

Some names have been changed. 'The Sex Inspectors' starts on Channel 4 on Tuesday at 11.05pm

Sexpert Advice

'You can talk about a sexual problem until the cows come home, but in order to get over it, you need to practise. And there may be deeper issues involved' Paula Hall, sexual psychotherapist

'Therapy definitely improved our sex life. It's more relaxed and more enjoyable. Neither of us is worrying about what's going to happen in bed' Jamie and Charlotte

'I'm a sex coach, not a therapist. I offer men who are already good in bed the chance to become exceptional. It is like sports training; I take people up to the next level' Sam Van Rood