Even by the usual standards of British sex scandals, this one is quite something. In March, the News of the World filmed Max Mosley, the president of formula one's governing body, the FIA, being whipped by several women in a prison-guard scenario - video footage of what Mosley has since described as his "unfortunate interest". The News of the World alleged that the video had Nazi overtones - that the women spoke to him in German and were pretending to be concentration camp guards. Mosley, the son of the British fascist leader Oswald, denies this. He says that it was a private party with consenting women and this week launched a case against the News of the World, saying that its stories were not, as it claims, in the public interest and had violated his privacy. "I've been doing [S&M] for 45 years and ... if it hadn't been for bribery and illegal acts, this wouldn't have come out," said Mosley, who added that he had kept his activity from his wife.

One of the things to have come out in court was Mosley's defence that his interest in S&M was not degrading or sick, and that the News of the World was out of touch. James Price, Mosley's QC, said: "It's not a surprise to me or to others who don't live in an ivory tower or a monastery, or, I am sure, to your lordship, to learn that quite a lot of people, men and women, have a fascinated interest in this sort of thing."

Nobody knows how many people engage in BDSM - the acronym for Bondage Domination Sadomasochism - but it is estimated that 14% of men and 11% of women have tried it. There are a huge number of BDSM websites, with everything from sites giving advice and instructions, to personal ads, to online groups where people arrange to meet. Next month, for instance, Unfettered, a voluntary group set up four years ago to run educational workshops and promote and defend BDSM, hosts Kinkfest in London, "a whole day of non-stop salacious stage entertainment, fantastic stalls, wicked workshops, superb speeches and special guests". Is everyone at it?

"I think there are people who wouldn't consider themselves practitioners of BDSM but who have experimented with tying themselves up with a dressing-gown cord or done a little bit of spanking here and there," says Pamela Stephenson Connolly, a clinical psychologist and sex therapist. "It comes up all the time in my work and people understand that what they're doing is playing out a form of BDSM. If the rules are observed, it is a perfectly reasonable part of the very broad tapestry of normal human sexuality."

Stephenson Connolly recently carried out a study of 132 BDSM participants to try to find out whether there was more prevalence of clinical disorders, such as depression, anxiety or psychological sadomasochism among those who practised BDSM. "The answer I found was no," she says. "Most of the assumptions that my profession has made about people who participate in BDSM is that there must be something wrong with them, but I didn't find that at all. What I did find is that the credo in the BDSM movement - and I'm not talking about people who don't know what they are doing - is 'safe, sane and consensual'. People would discuss what was about to occur, and talk about their limitations. I discovered tremendous detail in how it is played and anyone who doesn't obey the rules is kicked out of the community and nobody will play with them again. People are very anxious to ensure that consent takes place."

Psychologist David Mirich and sex therapist Neil Cannon have been conducting a major study into sadomasochism in the US, and believe that BDSM behaviours are much more common than is popularly thought. As Cannon says, "there are many people who have used a silk scarf to tie up someone's hands, or used a feather to tickle someone, or pulled their partner's hair - you see references on TV and in magazines to 'rough sex', but I don't think that the people involved see themselves as participating in anything that could be called sadomasochistic". The reason that we don't hear more about this, they say, is that "even with a great friend or a psychologist, people are very hesitant to talk about their inner sexual world".

Mirich has studied about 220 BDSMers and says that he "was fascinated by this historical and popular assumption that these individuals were traumatised in some form or fashion in their early childhood ... What I found was that most of them would tell me that, no, they were pretty much in an idyllic family situation as a child, that there was no sexual and physical abuse - in fact, there was no spanking."

I speak to a man known as Sir Guy (who is not really knighted), who has been involved in the BDSM scene for more than 20 years and is following the Mosley case with interest. He says that BDSM has gradually started to gain acceptance in the mainstream, and that he now has "thousands of inquiries and hundreds of people come to events ... It's mainly middle-aged, middle-class, middle-income people. Almost all are professionals - we have engineers, doctors, nurses, teachers, solicitors, accountants. There are a couple I know, both primary school teachers, who like spanking. It's Middle England, very, very normal and well-adjusted. We get the odd wild-eyed person, but we weed them out early."

This all seems positively tame compared to what other people seem to get up to. Unfettered has advertised workshops on "needle and medical play" where, among other things, female and male catheterisation is demonstrated. The advert reads a bit like a Women's Institute newsletter advertising a flower-arranging course. "If there is time," it reads, "Mistress Demonic will also run a short skin stapling demo as well."

"We live in a world," says Susanna Abse, the director of the Tavistock Centre for couple relationships, "where there is a wish to normalise such things." More and more people, she says, are prepared to experiment sexually, and are being encouraged to do so. "We need to ask ourselves questions: what is perverse?" she says. "Does the ordinary man in the street think this is 'normal'? I don't think we're having enough of a debate about what we think might be linked to some level of distress rather than sexual choice.

"In a way, what people do in the privacy of their own bedroom is absolutely fine. In fact, we might understand S&M as a sensible way of containing very strong feelings, such as aggression, if it's consensual. But, on the other hand, there's another kind of S&M that exploits other people, or makes people feel very disconnected from their experiences, where the relationship has broken down because of the activities, or where people feel betrayed." What is important, Abse argues, is not the activity itself, but, "what feelings are these activities managing? Does it represent some level of distress in the person?"

If BDSM is becoming more popular, you can blame the internet. "You name a fetish and there will be an online group or a website for it," says Derek Cohen, chair of the Spanner Trust, an organisation "to defend the rights of sadomasochists". "I'm gay and when I was growing up, there was no gay visibility and you felt like you were on your own and there was nobody around like you." It was the same for people into BDSM, he says. "Now, if you're into getting excited by people popping balloons - and I'm serious, there are people who are into that - you will find like-minded people. And you can actually make dates with people and go and play. The community aspect makes it safer because the people are known. If someone is going to cane me, do they know when to stop, do they understand hygiene issues, do other people know them, for instance?"

Is BDSM is becoming more popular? "It's hard to say," says Cohen. "I think it is becoming more evident in mainstream media and becoming more acceptable in a titillating way. You see big billboard adverts of women in leather.

I think that is bound to encourage more people to feel happier about the fact they have those desires and want to act them out. We are much more accepting of the diversity of people's sexuality generally, although there still is a lot of disapproval about BDSM."

Perhaps it's because many people can't understand the point of it. I ask him to explain. "There are two sides to it - a physical side and an emotional side," he replies. "I would say 10%-15% of the population find the idea of power and control quite exciting, whether that's being tied up or being told what to do - a lot of people like that. A lot of people have fantasy sex - it's a way of escaping their lives and being a different sort of person - and some of those fantasies involve being slaves or being dominant. The physical side, I put it this way: imagine having sex with someone who runs their fingers down your back; then they do it harder and start using their nails and it gets a bit painful. It's a continuum from that for people who are into it. The fact that I like a certain amount of pain doesn't mean that it stops hurting when I go to the dentist - it's a different context, it's not eroticised."

Not many people may be aware that BDSM is actually, technically, against the law (part of the News of the World's defence is that Mosley was taking part in a criminal act, because he bled when he was spanked). Or, as Sir Guy puts it: "If someone comes to me because they want a caning, we could be locked up for it. The laws in this country are ludicrous."

The Spanner Trust was set up in 1995 after 16 gay men were sentenced to prison or fined for engaging in consensual S&M activities. "The law established that you can't engage in sexual activity that left marks or bruises or injuries that are 'more than transient or trifling'," says Cohen. "But in the Spanner case, none of the injuries were serious - any cuts or bruises healed within days - and it was all consensual. But you can get in a boxing ring where people can die or suffer severe brain damage, but you can't consent to BDSM. I always say that when sex comes in the door, common sense goes out the window. It's restricting people's right to enjoy their sexuality. BDSM doesn't cause a large amount of injuries because people aren't intending to seriously hurt people, they are intending to have fun. You're not doing it in a hostile way."

Sir Guy says the people he meets who are into BDSM are "very normal, well-adjusted people. They are better company than you'd find at a stamp-collecting society or line-dancing group. They are more articulate, aware and caring than most people I have met. People have found it time and again - if someone goes through a divorce or bankruptcy or difficult time, it's their friends from 'the [BDSM] scene' who rally round." Sounds cosy, really. Apart from the whipping.



Read Jon Henley's history of S&M here