TRACEY Cox may be a seasoned sexual commentator, but when she first sat down to watch footage of couples copulating on camera, she admits she didn't quite know where to put herself.

When the first series of Channel 4's The Sex Inspectors aired last autumn, viewers saw only momentary flashes of surveillance camera sex: thermo images of men and women morphing into each other, wrapped, as one critic put it, ?in a Ready Brek glow?. They saw Sally, curled over Pete, performing a frantic blow-job on what looked to be a motionless corpse. Here was real sex on television: what ordinary people look like, lost in the dark hours, ridiculous and animal-like.

Spare a thought for co-presenter Cox, who was exposed to several hours of unedited filming. She may have written straight-talking books with unambiguous titles like Hot Sex and Supersex, but she was taken aback by the raw reality of watching people actually doing it. "But that's what I like about The Sex Inspectors, " she insists.

"Very rarely do you get to see real people having sex. You get to see people acting it in movies and it's crap. Real sex is very reassuring. You think, 'Oh my God, they also do that', or - 'Their tummies wobble, too'."

Cox's swanky Richmond pad - which features walls hung with Helmut Newton nudes and a voluptuously curvaceous glass table - looks as though it might have been designed by the makers of smart erotic toys.

Her body language couldn't be more open, but then she's the expert: a psychology graduate who acted as flirting coach on BBC2's dating programme, Would Like To Meet, and who now offers couples onscreen advice on bedroom techniques.

Big sisterly, frank and indiscreet, Cox punctuates the interview with responses to the text messages that keep whistling into her phone. "Oral-sex boy, " she says glancing at a message from one of her exes: the one who arrived clueless at cunnilingus and was packed off with "the best oral-sex skills in the world". The 43-year-old divorcee has a taste for younger men. Currently seeing a 29-year-old, she's nicknamed all the recent men in her life - there's "cop boy", "boring boy" and "panto boy" - a habit she warns me against since "you never take them seriously".

Cox, a former editor of Australian Cosmopolitan, is a journalistic authority on matters of the heart, as well as the libido.

Her interest in the subject is deep-rooted.

When she was 15, her father left her mother following a 10-year affair: one that had prompted her family's emigration to Australia. "I thought: 'Powerful emotion, this love-sex thing'. My sister was working in family planning too, so I was getting a lot of information. Then I went to university to do journalism, and discovered psychology. I am absolutely fascinated by sex, relationships and emotions."

Initially, she was reluctant to become involved with The Sex Inspectors; the producers took a year to persuade her. Now, however, she defends the programme like a crusader, deflecting accusations about sensationalism or potential harmful effects on participants. "Everybody gets hung up on the fact that people have sex, " she says, "but out of the six couples we did last time, two of them had a lot of sex on camera; the other four didn't really. I think the couples were clued up enough to realise the programme was actually not about having sex on screen.

It's about sharing their sexual experiences, getting something out of it, and helping others in the process. They're very brave - forging the way for everyone else."

Exhibitionists apart, why would anyone agree to allow their sex lives to be so glaringly exposed to the nation? Even Cox admits she's not sure she'd be willing to do it. Response to the first series of ads seeking participants wasn't overwhelming: only 40 couples were interviewed. What's striking is the sheer normality of the chosen six, and - compared with other reality TV shows - the programme's toned-down tenor. There is more exhibitionism in five minutes of unedited Big Brother than in a whole episode of The Sex Inspectors.

Cox stresses that the couples knew from the start what they were getting involved in.

Some didn't even have sex on screen. Nicky and Andrew from Brighton, for instance, bunked off to a camera-free room whenever they fancied a bit of action. Charlotte, meanwhile, who volunteered 15 hours of on-screen action with her partner, Jamie, later said she didn't regret a minute of it.

"Why should I? Sex isn't something to be ashamed of, or to be thought of as dirty."

People with their own agendas for taking part in such programmes - for instance, an ambition to become a model - were avoided during selection. And in fact, once the programme was screened, the most serious complaint was that actually, it was a little dull, just not quite titillating enough.

Cox says she was even accused of being "too prudish, too schoolmistressy".

Much of the controversy centred around the complaint that Cox isn't an accredited sex therapist (there is no proper accreditation for sex therapists). Cox ascribes her expertise to experience: "I've had lots of different relationships. I think it's quite good that I'm not a sex therapist in a way, because I'm looking at it much more from a layman's perspective."

Her approach to sex isn't particularly radical. She ascribes her success in the field to the fact she likes "good-girl sex"; in other words, her sex life is pretty average. She doesn't go in for threesomes, swinging or anything "kinky" and is almost absurdly traditional - advising, for instance, that women delay penetrative sex until after six weeks of dating. "I'm quite sensible with love, because I see people hurt the whole time.

Having sex too early causes lust blindness."

Cox is a sexual liberaliser with her foot in the mainstream. Her books - sex etiquette guides - sit like Sex And The City, at the softly frothing blockbuster edge of a wave started decades ago by Alfred Kinsey, whom Cox applauds for having the bravery to bring a taboo subject out in the open.

In contrast to the controversial US scientist, Cox seems almost homely in her sexological wisdom. She is a journalist, who sometimes seems to be running the gamut of magazine babble: "What star sign are you?" she asks at one point. I tell her Scorpio. "Oh my God, so am I. I bet you have a high sex drive."

When I tell her the ice-cream licking technique she demonstrated on the show wouldn't be my thing, she points out that about 70per cent of women like it. Of all the "millions of books on sex" she has, Alex Comfort's The Joy Of Sex remains her favourite.

"It was way ahead of its time: such a lovely, broad liberal, whatever-feels-good-andisn't-hurting-anyone book, written by a guy who really enjoyed sex.

"Now, I know he enjoyed sex because he had an affair with someone who went to my hairdresser's and she said how great he was."

People are always asking whether Cox's role as a sex guru scares men. And to an extent, she admits, it does. "But that depends on the guy, doesn't it? And it's not just to do with sex." She probably has more money than her partners . . . and she knows about psychology. "They're screwed, aren't they?"

Cox stresses that The Sex Inspectors isn't a deep psychological programme. "It is, " she says, "a light entertainment show." If it has a message to convey, it's that communication is all-important. "People worry about upsetting each other, " says Cox. "It's hard to say to someone, 'I don't think you're very good in bed', or 'You don't turn me on'.

So they shut up about it because they don't know how to deal with the issue." Which is a pity, says Cox, since in fact, there are plenty of ways of addressing such problems. If the programme works at all, it is simply by provoking viewers to put their cards on the table and speak up.

According to Cox, most couples' sex problems boil down to one of two things.

The first is mismatched libidos; Cox wishes people would choose partners more carefully. "High sex drive people need to go out with high sex drive people and vice versa."

For while highly sexed people experience desire as "a raging fire", those with more modest urges may simply be aware of "a tweak of the embers". Yet such mismatches aren't hopeless, and for those who discover their lack of compatibility after they've got hitched, Cox suggests: "The first thing to do is remove any blame. Then you try to make the sex as good as possible, because if the sex is fantastic then you want to have more of it, don't you?" The other contentious area is kids. "Married couples' satisfaction goes down when they have children, " says Cox, "and it goes back up when the kids reach about seven years old. So if you can hang in there, you're OK."

Cox doesn't have children. Diagnosed with cervical cancer at the age of 30 while married, she was warned by doctors not to delay becoming pregnant. "They told me if I wanted to have kids I had better bloody start now." This made her take a long, hard look at her relationship. "And it made me realise that I probably wasn't going to stay with my husband. I loved him to death but it just wasn't working."

If she was to do her 30s all over again, Cox would "try to factor kids in", though in fact, motherhood still isn't out of the question.

Last year she stopped using condoms with a steady boyfriend and became pregnant.

When she miscarried days after arriving in New York, she realised she had wanted the child. Now that she knows she can conceive, she's not sure what to do. After all, she is currently in a short-term relationship with a much younger man. She likes her life as it is, with all its jet-setting and endless deadlines, and isn't sure she wants to give that up.

It's one of those commonplace ironies that this relationship guru, so capable of doling out advice, is stuck on such a fundamental question. She flips back into her agony aunt role. "I hasten to add that the having sex without condoms was within a monogamous relationship after we'd both had tests."

She may have deflected it with a joke, but the issue is out. Cox is by no means a woman who has it all.

The makers of The Sex Inspectors are looking for Scots couples for the second series.

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