Men may promise to forsake all others but how many keep those marriage vows? Almost a quarter of husbands in Britain have had an affair. The Argus goes undercover to investigate a website which invites married infidelity - and sees who's cheating in Sussex.

A Very Decent Guy from East Grinstead, 32, is "bored in marriage and looking for some spice".

Billy from Bexhill, 52, may be carrying "a few extra pounds" but he is "up for anything".

Fly By Night, a pilot from Haywards Heath, is searching for some fun when he's not cruising at 32,000ft and Chablis-loving, Brighton toff Dandy 7 is after someone classy. He wants to share Jacuzzis and Serge Gainsbourg with a woman who appreciates, "the sensuality of whispered words, especially en Francais".

While Dandy 7's greasy lines make me want to arm myself with a can of oven cleaner, it is different strokes for different folks and at AshleyMadison.com they come in all shapes and sizes.

At first glance, this US export seems like any other internet dating website.

But Ashley Madison's members all have something in common - they're married.

While we might tut at the thought of adultery (a YouGov poll found 84 per cent of us believe it is wrong), we don't always practise what we preach.

In the UK 22 per cent of married men and 15 per cent of women have had an affair and, according to the people at Ashley Madison, 35,000 of their 1.3 million members hail from Britain. Apparently their tagline, "When monogamy becomes monotony", has struck a chord this side of the pond too.

Intrigued, I pay my $55 (about £28) and pose as Mrs Slocombe, a 30-year-old from Brighton with the message "Hello, sailor" - saucy but not slutty, I think.

There is also the option to submit a picture. To avoid revealing my identity, I send off a snap of a cat which can play the part of Mrs Slocombe's infamous moggie. It is rejected on the grounds it doesn't resemble me.

The site's founder, Dan Morgenstern, has claimed members include housewives and movie stars and are "real people with real feelings... leading lives of quiet desperation".

While it's unlikely George Clooney or Brad Pitt lurk among the 106 quietly desperate men in East Sussex, or that Cate Blanchett is one of the four women chasing married men in the area, there's no harm in looking.

My female competition includes a "musical nymph" in her late-50s seeking a "discreet liaison with a kindred soul", a 37-year-old craving "attention" and a 33-year-old "looking for some fun".

While my rivals have kept their profiles brief - I up the ante saying "anything goes" and that I'm looking for discretion, good personal hygiene and imagination. I go to bed and wait.

The next morning my inbox has seen more traffic than the M6 on a Bank Holiday weekend. I have 36 messages. The first is from an imaginatively titled "8-inch Pete" from Brighton, who in reality is more likely to be 5-inch Pete. A 45-year-old of few words, the few he can muster include "oral sex" and "Don Juan".

Mistafit, 41, from Worthing has posted a picture himself at work with what appears to be his children's artwork in the background. Describing himself as a "nice guy looking to fulfil some dreams" he insists, unconvincingly, "This is not the sort of thing I usually do. I need to realise a few dreams and fantasies and find a lady who can hit the spot and add some spice to life."

Jollifier, 50, from Horsham isn't playing hard to get. In the space of an hour he has sent three emails, access to his private pictures and the toe-curling promise, "I'm going to rock your world".

He has the stare of Charles Manson and claims to be muscular - but from his picture it seems likely the only thing he's hiding under his leather bomber jacket is a machete. He emails again and suggests I discover his "gr8"

sense of humour. I decide I'd like to see out my 31st birthday, so don't reply.

Further afield there is Dominator from Weybridge. He has sent some very unattractive pictures of himself spilling out of cheap, women's underwear, which is likely to score him little other than a bad bout of thrush.

But by far the most frequent, and explicit, visitors to my inbox are highly-sexed males from the States.

Pick-up-man from Connecticut's opening gambit is straight to the point - "I'm very oral" - while Ezy from Alberta (also a big fan of oral) looks like a washed-up Jon Bon Jovi. The 41-year-old is into threesomes and sends me pictures of himself grappling with his very generous manhood.

While it's hardly subtle, at least I know he's only after one thing.

With so much interest I decide to concentrate my efforts on Haywards Heath pilot Fly-by-Night-747, fat but fun Billy from Bexhill and Petrus from East Sussex.

I start by emailing all three a "wink", to show I'm interested.

Within hours I have an auto response from my 5ft 10in muscular pilot.

It appears the eagle will not be landing in Brighton.

It reads: "I received a wink' from you and have checked out your profile. Regrettably, I am not interested. Thanks for responding and Good Luck!"

Surprisingly hurt at being rejected by a man I've never met, I console myself with the fact there are plenty more philanderers where he came from.

Billy is next to reply. By his own admission, he is "carrying a few extra pounds". He is turned on by my "willingness to experiment" and is very preoccupied with Mrs Slocombe's feline friend, but not in the wholesome way of John Inman's Mr Humphries in Are You Being Served.

Petrus, on the other hand, is a 39-year-old company director who works in the capital. He is 6ft tall, has good grammar and sends a lengthy email, which he admits was "more difficult than writing a CV".

"I am healthy, clean and well-groomed.

In short - I try and look after myself. I'm told that I look younger than I am and I am blessed (or cursed depending upon your point of view) with a relaxed and laidback personality and a half-decent sense of humour. I am well travelled, educated and would like to consider myself a gentleman'."

He sounds shockingly normal and after several emails he tells me he has been married for 16 years, has two children and has never done anything like this before.

His excuse? "I love my wife but we have grown apart," he writes.

"We've tried everything to get our relationship back on track but nothing has worked. I don't want to leave her but I need something more.

"Life is for living, we only get one shot and sometimes you just need to grab it and run with it. So here I am."

As we email I find myself almost feeling sorry for Petrus. Among the other men who email, the reasons - or excuses - for visiting the site are usually boredom. They are only very occasionally unexpected. Extra Long Tongue from Worthing wants to keep things strictly cyber - he says his wife is ill and can no longer have sex. And Flying Scotsman from outer London loves his girlfriend but she hates sex, so he is here on Ashley Madison, apparently with her blessing.

By the end of the week I've had nearly 140 emails from men all over the world. Admittedly, many of them are freaks.

The biggest of them all has to be 43-year-old Mistofflees who says he's from Hampshire but looks as if he has just escaped from Royston Vasey. He is into bondage, wears a goatee and sends photographs of himself prodding sausages on a barbecue and another standing outside a church.

If there's a subliminal message hidden in there I don't want to know what it is.

Occasionally there are men such as London-based Italian Stallion, who, as Paris Hilton would say, is "hot". He sends pictures of himself sunbathing and posing with a Lamborghini - which he insists he owns.

Dwane, 25, from Birmingham is a Thierry Henry lookalike who gets straight to the point, "Want some black?" - the answer, if I were single, "probably".

With so much attention, it's hard not to feel a little flattered but, according to psychosexual therapist and principal counsellor at Brighton Relate, Carol Buck, this is exactly where the problems start.

"Even the process of sending an email to an old friend is exciting.

What's going to happen? When will they reply? It gives you a rush of excitement. It really relies on that excitement and the element of risk needed to increase the thrill.

"It has been said cybersex is the crack cocaine of sexual addiction. It gives you a real high and you get a low when you're not doing it."

What about people out there, such as Petrus, trapped in loveless marriages and possibly looking for something more meaningful than a casual fling? Carol makes a very valid point: "If these people are looking for a relationship, a soulmate, why don't they go to match.com, why do they go to a site that is obviously about sex?

"On what basis do you believe what these men are saying? There is always an element of risk when you contact a person you know nothing about.

"There can often be a big discrepancy between who people say they are and what they actually are.

Meeting someone face-to-face in a pub, you can build a picture.

In cyberspace you don't know who you are dealing so you are vulnerable.

"In North Carolina a woman met up with a man she met on the internet and he murdered her."

With that sobering thought I email Petrus and Billy and say I'm going on a long holiday.

As I terminate my membership I receive one last email from Billy: "Come on Mrs S. You know you want to."

No I don't know that I do.

Adultery or just a bit of fun? Leave your views below.