"I had no idea there were other women and men out there who were as lonely in their marriages as I was," she says. "I've been married for 20 years to a man who is a lot older than me. We have a teenage daughter who, for many years, has been my focus. My husband and I have not had a physical relationship for years. I hadn't felt confident or attractive for ages."

So she joined the agency. Although the first few dates were "nerve-racking", Melanie has had a couple of affairs over the course of a year. "Some I haven't had a spark for; some have become good friends. I'm not doing anything that turns my life upside down. But I'm a woman: I need affection; I love someone noticing what earrings and underwear I wear."

How does she provide alibis for her husband? "I go out with girlfriends a lot," she says. Last weekend she stayed over with someone: generally, she meets her current lover for dinner in an up-market London hotel, a situation that might lead to an evening in a hotel room. "It's for companionship, intimacy and sex, in that order."

The point of the agency, Loving Links, is that as everyone is attached, the likelihood of becoming emotionally involved is negligible. But Melanie did fall for one of her dates, and the feeling was mutual. "He was my sort of person," she says, "but suddenly he felt really guilty and backed off. We said goodbye, lovingly. I miss him. But it could have become dangerous."

Is she worried about being rumbled? She says she once got back in the early hours after a date to find her husband waiting. "I had to think on my feet. I was worried we'd have a confrontation." It didn't happen and she continues to meet her current lover, as she has done for two months, and is very fond of him.

Alison, 42, lives in London. A good-looking woman who dresses in Nicole Fahri and Whistles, she works in publishing and is having an affair.

"I feel it could be my last chance before settling into bland middle-aged married life. I get pangs of guilt, but I don't feel that bad about it. My husband either knows and is turning a blind eye, or doesn't notice the difference. If the former is the case, then fine. If the latter is the case, then I feel vindicated."

Rebecca, 45, is a married lawyer. Hers is a different proposition. "I've never believed in monogamy," she says. "It's not a natural state." Her husband knows this. She has been married for 20 years, has a daughter of 19, and uses an extramarital dating website.

"The work-based relationship is dangerous. Meeting someone in a bar isn't my scene. People at dinner parties are off limits; they know someone you know." As a working woman, it's easy for her to perfect her trysts. "I often have meetings; it's easy to maintain."

So does she have itchy feet? "I've never had non-itchy feet. I'm not bored with my husband — we're happily married. It's not for want in my relationship. I like the chase." How many men has she slept with? "More than the national average. Or as it says in Four Weddings and a Funeral, "more than Lady Di'."

Romance and love aren't goals for Rebecca. "The L-word is to be avoided at all costs. We acknowledge we need the outlet, but we don't want to go to B&Q on a Saturday with them." While she does not like "slobbishness or overweight men", she is not interested in sex alone. "The outcome of the meeting is physical but I don't see it as simple lust. There has to be an understanding too."

She never feels guilt — "a pointless emotion. If you want to do something, you accept what follows" — but she believes fidelity is important. "I'm married and want to stay that way. I just don't see sexual activity as part of that fidelity."