Richard Martinez remembers with absolute clarity the first time he was paid to track a cheating husband. It was in 1999 and the client was a pregnant woman who was convinced that her husband, a businessman who owned a phone bureau, was having an affair. For four nights Martinez used his motorbike to follow the man in his car, jumping red lights and driving at high speed, but found him doing nothing wrong.

He was embarrassed at taking money from a pregnant woman and offered to pull out of the job. But she insisted that he carry on and explained that the fee was coming out of her husband’s company: he was paying for his own surveillance. On the sixth night Martinez caught him red-handed. He followed him to a place where he met his lover and photographed him kissing her “intimately”.

When he phoned to break the bad news to his client she had one more question. Could he get the other woman beaten up? Martinez found himself explaining that this isn’t what private investigators do.

If a recent report is to be believed, however, what private detectives do do is an awful lot of “matrimonial” work. The survey by Grant Thornton financial advisers claims that in 2006 nearly half of affluent divorcing couples in the UK engaged the services of a private investigator to confirm or rebut their suspicions of infidelity. Given that the figure for the previous year was just 18 per cent, and that since the 1960s it hasn’t been necessary to prove adultery for a divorce, this is a remarkable jump.

It may be that we are becoming less trusting or more savvy about the financial stakes involved in divorce (a wife might achieve a bigger settlement if she has been betrayed). Some investigators are sceptical of Grant Thornton’s figures, claiming that not only do quickie divorces make spouse-snooping less not more essential, but that technology (eg, being able to take photographs on a mobile phone or to buy spy gadgets for computers) makes people capable of doing their own sleuthing. But a number of recent cases have allegedly involved private investigators.

Ingrid Tarrant, the wife of the TV presenter Chris Tarrant, hired a private detective to follow her husband over several weeks. The millionaire Matthew Mellon is currently on trial accused of asking a detective agency to hack into the computer of his former wife, Tamara Mellon, the founder of the Jimmy Choo shoe empire. He denies the charge of conspiring to cause unauthorised modification of computer material.

For Martinez, the growth in this type of business has been palpable. Since he started the Expedite Detective Agency (www.ex-da.com) in London in 1999, he says, it has trebled. He doesn’t know whether this is because of advertising, word-of-mouth recommendation, or a burgeoning social trend, but he now employs several staff and describes his line of work as “lucrative”.

He started by completing a diploma in private investigating after leaving the RAF where he says that he served for nine years as a flying officer. “I advertised locally in 1999 and was inundated with calls,” he says. “And that was just from a single line in Thomsonand Yellow Pages.

“It tends to be seasonal – midsummer and Christmas are busy.” Why is that? “In summer ladies perhaps (dress) more provocatively. At Christmas there’s more alcohol around. Maybe partners feel that there’s more temptation.”

The most common request, he says, is for him to provide proof of infidelity in a divorce case. He uses mainly photographs and video footage and aims to find three occasions of the “target” with his or her lover. Women are more likely to hire him than men and in most cases their suspicions are correct. By the time someone picks up the phone to a private detective they are usually certain anyway. When he takes on new business he warns clients that they might not like what he finds. There are normally one of four reactions: disbelief, anger, distress or total impassivity.

Unsurprisingly, people do not tend to shoot the messenger bringing the bad news. “I thought I’d get that but I don’t. Perhaps because I sympathise with them – everyone has experienced relationships going wrong – they know that I’m there to help them. I tend to build up a rapport.”

There is certainly no lack of private investigators to hire: it is estimated that there are between 10,000 and 15,000 detective agencies operating in Britain many specialising in domestic surveillance. Woman’s Instinct, for instance, based in the North West, offers special rates to monitor office parties or social events and boasts that “if your partner has a car then we can track where that car goes for 24 hours at a cost of just £50”.

Alison Byrnes (not her real name), 38, from Leeds, used such an agency when her long-term partner became distant and irritable and had many unexplained nights out while denying an affair in 2004. After six months she decided to hire an investigator. It took him just 48 hours to get the evidence – her partner kissing a woman passionately in his car before disappearing into a hotel with her for three hours. It was a woman from his office but he had said that he was out of town attending a client’s birthday party.

“I knew in my heart he was having an affair,” says Alison, “You don’t live with someone for six years without knowing when they are getting sex elsewhere. But he had been so insistent that it wasn’t true, saying I was neurotic, unreasonable and that I’d drive him to an affair, that I worried I was going mad.

“When the investigator came back with the photos of them all over each other it felt like being stabbed. But then I felt relief and self-righteousness that I was the wronged party – not him. Confronting him with the pictures might have been a miserable victory, but at least it was a victory. It was £200 well spent.” Unsurprisingly, they separated.

The apparent increase in the number of people using private investigators to spy on their spouse might well be to protect their peace of mind. Stephen Grant, a partner with Grant & McMurtrie private investigation practice in Edinburgh, says that recently there have been many cases where a man, for example, has told his partner: “I’m leaving you, but there’s no one else involved.”

“I would say that in 90 per cent of such cases there is someone else, and the wife will come to us with her suspicions,” Grant says. “It may not help much in their divorce, but they just want to be able to say to their families that the marriage didn’t break down because there were faults on both sides, but because there was someone else.”

There are myriad gadgets and technology software available to spy on partners. Trackers with magnetic bottoms placed on the underside of cars, software that records every tap on your partner’s keyboard, or retrieves his or her deleted text messages. Even Oyster cards, the London travel device, can be used to follow its owner’s movements. Anyone who takes the card to a machine on the Underground, or keys in its serial number on a website, can get a read-out of every journey taken in the past ten weeks.

Martinez, 37, finds that polygraph lie-detector tests are also extremely popular. Incredibly, many husbands agree to submit to the test. Martinez visits the couple’s house (occasionally they will come to his home in South London) and he asks a series of questions preprepared by the wife while testing the target’s blood pressure and vocal chords when he answers. He has never been asked to conduct a polygraph on a woman – it is usually a wife’s ultimatum to her husband: “Have the test or I’ll leave you.” The majority of tests suggest that the husband is lying. Martinez says: When you lie the forming of saliva is restricted to the mouth and blood flow is restricted to tonsils and vocal chords,” he says. “You can’t disguise the change of frequency. I never give the results in front of the couple, I always phone later and tell them. I used to, but they used to start arguing in front of me.”

Martinez is certainly convinced that his industry has right on its side, even when he organises “honeytraps” – engineered situations in which a woman or man employed by the detective agency is sent into a bar or club to test a spouse’s predatory nature.

The bait, he says, is trained to ensure he or she doesn’t stray into the realms of entrapment. “A girl might drop her bag and spill some things next to the target and smile at him and it’s up to the target to choose whether to respond,” he says. She cannot, for example, offer him a kiss or sex. After the initial “hello”, she can react only to what he suggests. “Thereafter the honey-trapper is reactive. She won’t overdo the questioning, she won’t touch him. It’s all for (the target) to decide.” The minute the target does one of three things – asks for her phone number, asks for a date or tries to kiss her – the job is finished. She then makes her excuses and leaves, possibly by pretending to go to the toilet and not coming back.

Martinez says that he believes the process is morally sound. “Don’t forget we are proving innocence as well as guilt,” he says. “We follow the guidelines and they have a chance to opt out. We don’t do it when the target is under the influence of alcohol. That wouldn’t be fair. We’ll wait and do it another time.”

He tends to arrange a honeytrap about once every four to six weeks. “It is becoming more popular. It seems to be at a time when someone’s going to make a big commitment to someone – maybe buy property with them, have a baby or get married and they might have heard rumours from friends that this person flirts. So we’ll be there with microphone and a colleague taking a video discreetly, which we provide to the customer.”

It is then up to the customer to decide whether their partner has overstepped the mark.

There are certain things that private investigators cannot do if they wish to stay within the law. For instance, they cannot hack into someone’s credit-card account to check flight or restaurant payments. They cannot take photographs intrusively on private property. Martinez says that he also cannot obtain addresses from car registration plates and has to say no to anything that involves contravening the Data Protection Act.

There is also a risk that they might be helping someone with ill intent. Martinez says that he always asks what the client’s purpose is – especially with men. “I don’t know, say, if they are stalking someone and want more photos,” he says. “If I can, I try to suss them out. But because there's a lot of money involved they are usually legitimate.”

He has been caught only twice: once when a husband realised his car had a tracking device on it (he cannot put the tracking device on the car himself, he can only sell it to the wife and then let her do it) and found Martinez’s business card in his wife’s handbag; and another who noticed Martinez following him and went home and quizzed his wife, who broke down and confessed.

As you might expect, he doesn’t expect the demand for private investigators to wane. The work comes from all classes and all cultures, he says. There is a Muslim who has paid him for three years to keep tabs on his Western wife. Many Asian women come to him, desperate for proof of their husband’s philandering. “Indian women fear for their livelihoods,” he says. “In their culture they can’t leave their partner and if they are seen as the guilty one they are disowned. I have had quite a few begging me to find proof. That’s when my heart goes out.”

Some might say that he has a romanticised view of his job when it is, after all, paid snooping. “I love it,” he says. “There is job satisfaction that I’m making someone feel better, catching out bad guys or women, helping society to keep marriage as something important. And the pay’s good.”

Whatever your view, people apparently have less reticence about using a private investigator. So if you are playing away it’s probably best to bear this is mind. And check who’s behind you.

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