One spring afternoon last year, Joely Brown put a cassette into the video player, sat on the sofa and started to watch an extraordinary seduction scene. Extraordinary because the 'leading man' was none other than her own husband of 15 years, Nigel, and the 'leading lady' his former secretary, Nadine.

Filmed secretly by a private detective, the footage showed the couple caressing in a hotel before moving on for a romantic meal. Ninety minutes long, this devastating indictment of Nigel's adultery confirmed Joely's worst suspicions.

Eighteen months later, the memory still infuriates her. She recalls: 'I got up from the sofa and was physically sick. Sure, I had hired a private detective to follow my husband because I suspected he was having an affair. But part of me thought I was just being paranoid, like Nigel claimed.

Like Chris Tarrant's wife Ingrid, who hired a private detective to tail her cheating husband, Joely took the same drastic action after niggling doubts about her partner's behaviour.

It isn't a decision which comes easily. Joely, 37 and the mother of two girls aged five and three, lived in a comfortable middle-class idyll with husband Nigel, a 39-year-old company director. Their five-bedroom detached home in Leicestershire is far from the heady world of secret surveillance and hidden cameras, and Jo is almost apologetic as she explains her desperate measures.

'Our sex life changed, too. Nigel stopped kissing me when we made love. Then one night when I walked into the bathroom as he was drying himself, he threw a towel around his body and told me to get out.

'I asked if he was in love with someone else but he told me I was paranoid. In the end, I couldn't stand it any longer and looked up a private detective agency in the phone book.

'The detective pointed out all the possible outcomes so that I was prepared. He stressed that Nigel might not be seeing anyone at all; or, if he was, it could be a woman, a man, a teenage girl.

'My mind was made up, though. Later that week, Nigel was going away for a work conference which he claimed involved a Thursday and Friday night stopover. I was convinced he was lying. I gave the detective details of the hotel where Nigel was staying and he said he would put Nigel under surveillance.

'When the phone rang at noon on Friday, my hand was shaking so much I could barely answer. It was the private detective confirming that Nigel had checked out of the hotel - and was being followed.

She says: 'I was just so angry. For month after miserable month, Nigel had made me feel so low about myself. He had almost convinced me that I was unstable and paranoid.

An extraordinary story? Not according to Jon Riding, the private investigator who filmed Nigel's infidelity. Riding has spent 20 years on the heels of cheating spouses - and claims business is booming.

'I recently had to advise a client that there was no point in me following her husband for the fifth time because he really wasn't up to anything he shouldn't have been.

'But other findings come as a real shock. A wealthy financier in London hired me to follow her fiancÈ when she became convinced that the regular hospital check-ups he had for a supposed illness weren't bona fide.

Riding claims he is hired equally by men and women. He says: 'One in three marriages now ends in divorce and affairs are rife. But for every client who races to the divorce courts when I provide evidence of infidelity, there's another who will stay with their partner and try to work things out.

Corrine Davies, a 34-year-old accountant from Derbyshire, is another woman who took the step of hiring a private investigator to follow her partner of three years.

Incredibly, Nick, a 35-year-old surveyor, remains blissfully unaware that he was shadowed by a detective - or that a devastating dossier was handed over to Corrine in January.

Corrine says: 'We have a computer at home and shared the same passwords . Then one night he announced that he'd changed the passwords to give us separate ones to our email accounts. It unsettled me but I tried to brush it under the carpet.

'Alarm bells started to ring, though, when Nick began switching his mobile phone off at home in the evening. When I challenged him, he said he was simply trying to switch off from work.

'I wasn't convinced, so when I went into the office at home one night and discovered he'd accidentally left the computer logged onto his email, I couldn't resist having a look.

'To my horror, I discovered that he'd been looking at dating websites and had even registered on one. In his profile he'd written that he was "looking for uncomplicated fun with a woman or women". What's more, he'd even had emails from women on the site asking to meet him.

'I was furious and decided then that I needed to hire a detective to find out exactly what he was up to. I looked in the Yellow Pages and chose a detective who lived close by. When I spoke to her on the phone, I was already starting to feel unsure, but she told me she could at least get the details of Nick's life that I was missing.

Nick was tailed by Helen Foster-Clark, a 35-year-old private detective from Manchester, who followed him to a busy pub. There, as Foster-Clark sat in the shadows, Nick met a woman.

'I recognised her as one of the women who'd sent an email and photo in response to his dating website advert. They didn't kiss and left the pub separately three hours later, but they were very tactile with one another, touching hands and looking into each other's eyes.

'For a few days afterwards, I was too angry to confront Nick. I wanted to wait until I felt calmer so that I could say exactly what needed to be said.

'It was a week before I finally broached the subject, just as we'd sat down to dinner. 'What the hell do you think you've been doing registering on a dating website?' I asked him. 'He was so shocked he literally dropped his knife and fork.

'Nick said how sorry he was and that he'd registered on the website one night because he'd been feeling a bit bored. I realised that I loved Nick and, as far as I know, he hadn't got as far as the "uncomplicated fun" he'd advertised for, so I made the decision to stay with him.

Only time will tell if the uneasy truce between Corrine and Nick will last. But according to Foster-Clark, our generation of have-it-all career women are determined to know-it-all, too.

She says: 'Fifty years ago, when women stayed at home, philandering husbands could get away with their misdemeanours. Women weren't financially independent and self-sufficient the way we are today - they relied on their husbands.

'But now an ever-increasing number of suspicious wives and partners hire me to follow their husbands. They have the money to pay for a private investigator and they don't fear the truth because they know they have the means to rebuild their lives.

'In fact, many younger women with high-flying careers and assets to protect hire me before they take their marriage vows to make sure their husbands-to-be aren't playing away .

'Around 50 per cent of my clients are female, and they generally have pretty sound suspicions about their husband's or partner's behaviour when they pick up the phone to me.

Foster-Clark claims 70 per cent of women who suspect their partners are cheating are ultimately proved right - like Anna Chadwick, a 41-year-old mother-of-two who hired a detective twice.

She recalls: 'When you've been with someone for 15 years, you know them inside out. So when Mike started dressing in trendy clothes and coming home late from work in the evenings, I just felt there was something wrong.

In fact her suspicions proved true. Within 24 hours of putting Mike under surveillance, a private detective saw him leave work at 5.40pm and drive to a country pub to meet a dark-haired work colleague.

Anna says: 'I was shown a video of them in the pub, their legs touching. When they left, they stopped and kissed passionately. Watching the film and reading the detective's dossier of events, I felt physically sick.

Within two years, Anna was divorced. She says: 'Mike refused to pay maintenance for the children, claiming he was broke. I promptly hired the same detective again, who sat outside Mike's parents house where his address was listed.

'Mike showed up in a new Chrysler - and later drove to a detached, newly-built house. We discovered that far from being cash-strapped, he had bought a new home with his mistress.

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