Infidelity How To Catch A Cheat...Mrs Tarrant investigates

When she thought her husband was having an affair, Chris Tarrant's wife, Ingrid, hired a private detective. Sarah Harris finds out how the professionals trap a straying partner

The Chris Tarrant love triangle has all the elements of a 1940s detective thriller: the errant husband, the long-suffering wife, the scarlet woman and the private investigator - except that the action took place in the commuter belt and the femme fatale was 50-year-old school teacher Fiona McKechnie.

It emerged last week that Tarrant's wife, Ingrid, hired private detectives to follow the Who Wants to Be a Millionaire? host and reportedly told them: "Nail him and I'll divorce him!" The surveillance operation failed, but the game show host was later spotted snogging a blonde in a winebar in Esher.

Paul Hawkes is a veteran private investigator, who has caught out numerous philanderers - he has 25 cases on the go at any one time. We make an appointment to meet at 2pm in west London. Sadly, it's too warm to wear my trench coat.

Hawkes, 47, has been a private detective for 29 years and runs his company, Research Associates, from a leafy residential street. The door is opened by his PA and a small black dog leaps around my ankles. "That's Diva - isn't she cute?" says Hawkes appearing from the shadows. She goes everywhere with me."

As we drive though Kensington, Hawkes outlines the tell-tale signs of a cheating spouse. "When someone is cheating, there is always some sort of break in their routine," says Hawkes. "The target's relationship with their mobile may have changed: they may have started taking it into the shower, or it is constantly switched off." Bad traffic it seems is one of the most commonly used excuses for being absent or late.

"There's a myth that the investigator knows everything," says Hawkes, "but it is the client who knows everything. When they confront their partner with their suspicions, they are usually told they are imagining things or that they are crazy, so they hire someone like me to provide them with some concrete proof." Their suspicions are usually correct with 80 to 90 per cent of his "targets" found to be unfaithful.

The clients are not just the desperate housewives of the classic film noir mould - around 35 per cent are men. His clients are usually what he calls, high-worth individuals. This is just as well as a team of three surveillance people can cost in the region of £165 an hour, with most investigations lasting two or three days. However, the cost of some of his more complex cases has run into tens of thousands of pounds.

Surveillance teams follow targets to parties or nightclubs, and will wait outside offices, restaurants and hotels. They take video, record conversations and snap pictures with the aid of hi-tech recording devices and cameras. It is perfectly legal for his teams to tap a phone or a room with the owner's permission and to record video or audio footage on public property, "as long as you're not poking your camera through someone's living room window".

When successful, says Hawkes, they often uncover details that are difficult for the client to see or hear. Hawkes describes one case where a suspicious wife hired him to set up transmitters in the family home when she was away on holiday with their two children. While she was gone, her husband invited an ex-girlfriend round for dinner. "It soon became evident that they were having sex in the house, which was devastating for the client to hear."

Another time he had to break the news to an Arab woman that her husband had been having an affair with a transsexual prostitute. She had no concept of what a transsexual was until he explained it to her.

Hawkes suggests that we take Diva for a walk. As we make our way across the park, I ask him if his job makes him feel the world is full of cheats. He tells me he still believes in honesty: "The only currency in a relationship is trust. Love, sex, children, finances - everything relies on trust," he says. "We are all grown-ups, and if a relationship isn't working and the trust has gone then there is nothing left. I'm lucky enough to have a wonderful relationship, and I know I can come home and feel very comfortable looking directly into my wife's eyes."