Who watches the private detectives?

An ordinary suburban street in Brentwood, Essex. In the distance, the sound of a power tool can be heard. I walk up the driveway, past a white van, and press the doorbell.

After some time, the door opens. Maureen, a striking lady in her sixties with a platinum bob and bubbly Essex accent, ushers me into the sitting room, and offers me a cup of tea. Her husband, James, a big, affable man, gets up from a floral armchair and shakes me genially by the hand. Maureen regards me coolly.

Meet Mr and Mrs Harrison-Griffiths, a husband and wife team who run a detective agency, Aitch-Gee Investigations, from their spare bedroom.

James is a retired detective chief inspector, who once led a murder squad in north-east London. Maureen is her husband's sidekick. In the spare room is a listening device disguised as a phone charger, and a pen that contains a camera. The anonymous-looking van outside turns out to be a surveillance vehicle, complete with a green jerry can in which to urinate. "Maureen thinks it's disgusting," says James, "but I like things the old fashioned way."

The world of private detectives is a strange one. Thanks to Glenn Mulcaire, Steve Whittamore and a host of others connected to the News of the World hacking scandal, private eyes have never been lower in the public's estimation. But they are not all hi-tech villains and grubby bin-raiders. As part of a Radio 4 documentary I recently met a few of the other estimated 10,000 investigators operating in Britain.

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Admittedly, many people I approached refused to speak. One or two agreed in principle, then pulled out. But, eventually, I found several who were willing to go on the record and they proved to be an eclectic bunch. At one end of the spectrum was an art-crime detective who has travelled the globe recovering masterpieces worth millions. And, at the other end, there was Mr and Mrs Harrison-Griffiths.

"The first time I went out on a job, I almost wet myself," says Maureen as we sip our tea. "We were chasing a cheating husband around the M25. Jim was driving at 100mph, and I was hanging out the window with a camcorder to get some evidence. I've never been so scared in my life."

"She wasn't so great to start with," James concedes. "On surveillance, she would stick out like a sore thumb. But I taught her the basics, and she picked things up quickly. These days she's a very effective investigator."

Aitch-Gee Investigations is one of the smaller agencies. The brunt of their business is split between tracing missing persons and "matrimonial", or catching philanderers. Several times a year, however, James has a chance to dust off his murder squad skills. When a death occurs that is unexplained, and the police investigation seems to be lacking, bereaved families turn to him for help.

One such case he is working on at the moment, he says, involves the death a few years ago of a 17-year-old girl who fell from the window of a tower block. At the time, the police released a statement asserting that "there is nothing to indicate that her death is suspicious".

According to James, however, the forensic evidence he found suggested that the girl had probably not climbed onto the ledge herself. The police, he says, had also failed to commission a "falls expert", who may have been able to ascertain if a third party was involved. James, on behalf of the victim's family, has been trying to "do the investigation more thoroughly this time".

"The police often hurry their investigations, particularly with all this cost cutting," he explains. And although James doesn't believe there is any excuse for sloppy police work, he concedes that, "the police have limited resources, and this means they don't always do an effective job."

Businesses like Aitch-Gee are often run by ex-police officers, who draw - sometimes too liberally - on their training and contacts. (Last month, three former policemen were arrested on suspicion of paying thousands of pounds in bribes to serving officers.) But James and Maureen are adamant that everything they do is legal. "You can photograph who you like in a public place," says Maureen. "We'd never get involved in anything dodgy."

Not all agencies rely on ex-police, however. Answers Investigation, whose offices are tucked away in a farmyard in Surrey, have 11 full-time and several part-time employees, all of whom are from civilian backgrounds. The founder, Nigel Parsons, has a marketing background, and sees a varied workforce as a strength. One of his specialities is to employ girls in their late teens and early twenties, who can go undercover, for example, as schoolgirls. He has even set up a website called teendetective.co.uk, which features profiles of eight young, female employees, and directs users towards his main site.

Olivia Ellenger, 21, is Answers Investigation's newest employee. She is an impressive woman. Although she could pass for a teenager, she displays a self-assurance and level headedness beyond her years.

"When I had my interview," she says, "they brought up my Facebook page to see how I reacted. Then they asked questions about my life, and said they would know if I was lying." Why didn't she just get up and walk out? "It's a very special industry," she says. "You've got to know you can trust people from the word go." The female employees do everything from tailing a subject around town to monitoring neighbours on behalf of a prospective homebuyer who wants to know if children run riot in the street or hold noisy parties.

At the back of Answers Investigation's offices, behind a door marked forensics in progress, do not enter I meet Andy Cross, who leads many of Answers' operations. He is a tall, well-groomed and rather opaque man. And he isn't exactly short of stories.

"You do need to trust each other, but it's not all sleaze and intrigue," he says. "The most fulfilling part of my job is proving people innocent, or finding long-lost family members. When someone is reunited with their biological mother and you've been part of that, it can be very moving." However, he admits that these cases are in the minority. "One of my favourite jobs," he says, "involved two employees who set up a rival company. Their boss suspected them of stealing his customer database, and called us in to investigate." Cross set up a surveillance team, and observed that the "targets" went out to buy sandwiches every lunchtime. So he set up a fake sandwich company, and put a leaflet through their office door offering a free introductory platter. The target made contact to arrange delivery. He was in.

"I went in with a red T-shirt and baseball cap, which contained a hidden camera," Cross recalls. "I gave them the sandwiches and asked them if they had any advice to give me as a start-up. Before long, they were boasting all about how they stole the database from their previous employer. I caught the whole thing on video, and that was used to convict them in court." He lines up a selection of his equipment on the table: a fingerprinting kit; cigarette box complete with listening device; and, his personal favourite, a Costa coffee cup with a hidden camera. "You have to be careful, though," he says. "If there's no Costa nearby, you'll give yourself away." Another easy mistake, he says, is to forget to turn off the flash when taking a covert photograph.

Cross's boss is feeling the worse for wear. He was working until four the night before, he says. He drinks coffee and rubs his eyes as we talk. "A lot of what we do is basically police work," Parsons tells me. "The cops have limited resources, and often just aren't interested." This is particularly the case with corporate fraud, and accusations of sexual abuse that are hard to prove.

"People who can afford it pay us to conduct the initial investigations," he continues. "If we are able to build a case, we hand it to the police. But not everyone has that sort of money." A sustained investigation can be expensive. Answers Investigation charges £54.50 per hour for surveillance, £65 per hour for "computer forensics", and up to £95 per hour for "specialisms" including fingerprinting and complex forensic analysis. These prices are per investigator, and sometimes large teams are required. Expenses are claimed on top. Parsons says his fees may start in the hundreds of pounds, and stretch into the tens of thousands.

"This month we opened two new branches, one in Sussex and the other in Hertfordshire. We already have one in Berkshire, which is flourishing. It's brutal to say it, but we're doing very well out of police cuts."

Many people are shocked to learn that the private investigation industry is completely unregulated in Britain. Although there is an Association of British Investigators, they have no teeth and membership is strictly optional; according to Parsons, less than 20 per cent of private investigators are members. Anyone can set themselves up as a detective. No licence is needed, and no inspections are made. "We have been supporting the campaign for regulation for years," says Parsons, "but it just isn't a priority for the Government. Even after Leveson."

Some private detectives, however, are already taking steps to avoid any potential regulation. One such person is Charles Hill. "I call myself a crime researcher," he tells me. "That way, if regulation ever comes in, I can dodge it." This doesn't necessarily mean that his methods are insalubrious, says Hill; he is simply a maverick, in the tradition of private investigators, and does not want his wings clipped by bureaucratic constraints.

For many years, Hill served in the art and antiques squad at New Scotland Yard, where he made his name by recovering Munch's The Scream in 1994. After that he "went freelance", which, he says, suited his "lone-wolf" nature. This was certainly effective. Among the artworks that he has recovered is Titian's first masterpiece, Rest on the Flight into Egypt, worth approximately £6million.

"I built up many contacts within the criminal world," says Hill. "Most art crimes are carried out by one of several families, and they trust me. So I'm able to negotiate the return of stolen masterpieces, for a reward."

Whatever the end result, shouldn't it be illegal for civilians to carry out police work? Parsons becomes very animated when I put this to him. "Much of the industry is made up of people who would quite happily hack phones, buy and sell private data, and all that," he says. "We don't do it. We stick to the law. When a case reaches a certain point, we notify the police." But this creeping privatisation of police work worries even him.

Parsons admits it's a "constant frustration" that his services are only available to those who can afford it: "But investigations cost money, so there you go. It's not that we don't care. But we are a business, at the end of the day."

The documentary 'Crouching Low, Hidden Camera: Life as a Private Investigator' will be aired on BBC Radio 4, at 4pm tomorrow, and repeated at 1.30pm on Sunday

This article also appeared in SEVEN magazine, free with the Sunday Telegraph. Follow SEVEN on Twitter @TelegraphSeven