Some people will do anything for money - torch cars, break windows, shoot somebody. And then there are the lads from the Get Back Agency, who take the light-hearted route to revenge
"We like to deal with the client one-to-one," says Dan, describing his thriving new business. "They're involved with it as much as we are ... I think we're answering the needs of a growing problem." He could be talking about interior decoration or selling mobile phones, but it's a much more primal urge that Dan and his partners' particular service industry aims to satisfy: the urge to get even.

Revenge may be sweet; it's also highly profitable. In these service-oriented times, getting your own back doesn't have to mean getting your hands dirty. Got an egotistical boss who needs to be brought down a peg or two? No problem. Just get the boys to make a few bogus phone calls. Noisy neighbours? Stop the offenders once and for all and get some bright spark to cut off their electricity.

Dan, 23, and his friends Richard, 27, and James, 23, set up the Get Back Agency last year. They will organise pretty much anything, from bogus letters and silly phone calls to a ton of manure in someone's front garden. The feel-bad sector seems to be brimming with commercial possibilities, as Dan, Richard and James can verify. "We do get a lot of weird calls from people asking us to do someone's kneecaps in," says James, who makes it clear that the agency always turns down "dodgy" requests. "There's so much publicity about not getting justice," says Richard. "People think they're entitled to it and they want it yesterday. Saying that, I'm quite happy to make some business out of it."

The Get Back Agency is one of several groups of willing avengers who appear in a BBC1 Inside Story documentary "Dial V For Vengeance", all of them making money, in one way or another, out of people's desire for retribution. At one extreme, there are villains who would consider murder for a price, at the other there is the couple who do a nice line in "crapograms" - making your feelings known through a plastic poo in a custom-made gift box.

Somewhere in the mid-range of the spectrum is the Get Back Agency, who like to bring humour and creativity to the job. Clustered around a table in a north London McDonald's, Dan, Richard and James, all advertising and graphics students, look like a trendy-ish, clean-cut boy band. Their typical customer is the bloke who reads Loaded - the man in the office who wants to get back at other men in the office. "Women seem to be more serious," says Richard. "They don't phone up for the fun, practical jokes."

"We tend to get lots of spurned women crying on the phone," agrees James. For about pounds 20 he can relieve their anguish, or at least make it more bearable. "A typical job would be, a wife finds out her husband's having an affair and he might also have had a vasectomy. We'll sort out a letter to him which looks like it's from the hospital saying there's been a problem with the operation."

Does he ever feel bad creating distress, albeit temporarily, for the vicarious satisfaction of a third party? "Nope. Never," says James resolutely. "As long as they're funny I don't really mind," agrees Richard. "It is creative - it's about having good ideas. Actually knowing you've pulled it off - that's when you get the buzz," says Richard. Their antics also stretch to newspapers. Last week, James was irritated by the tone of a journalist from the Sun and gave her a bogus address for an interview followed up by one of his special letters.

But the stunts that really give them satisfaction are the bigger, more theatrical ones, like the time they caused havoc in a posh health club for a couple who were furious that their membership application had been rejected. Get Back hired a rather large lady from a casting agency, "Mrs RolyPoly", to play a prospective visitor and filmed her causing a rumpus in the swimming pool before being thrown out. They even organised a mock demo outside the club, with protesters chanting "Lard Is Life" to an irate health club manager.

Richard beams with satisfaction at a job well done. Another favourite was carried out for a customer whose neighbour's dog, three doors down, kept pooping in his garden. "We bunged a mass of elephant dung in the owner's garden at three in the morning. It was as easy as anything. I like to think those are the best sort - nobody gets hurt and it's quite good fun."

After they stop chortling about elephant dung, Richard looks thoughtful for a second. "I don't want to get too philosophical," he says. "But the trouble is everybody is out for themselves - they aren't prepared to work things out with each other. People don't put up with things as much as they used to."

While Get Back prefers to keep things light and zany, Kenny (not his real name) peddles a much grimmer line in revenge. Hunched over a drink, he sizes up just how far he'd go for pounds 20,000. Murder? "Mmm. For pounds 20,000 I'd obviously be thinking," he says, smirking. He doesn't want to go into details about his previous work, but says he spent some time on building sites and driving lorries. His appearance is sinister: Seventies' sunglasses, scars, slicked-back black hair and a long trenchcoat. "The main objective is to give them a fright and teach them a lesson," he says. "If someone was going to give me pounds 10,000 or pounds 20,000 then my sense of humour goes out of the window."

He gets more women calling than men; many of them, he says, are middle- class and the victims of domestic violence. "It's often women who aren't used to violence," he says. "They're shocked that their husband's laid a hand on them - usually for the first time."

Kenny, in his 50s, works alone through word of mouth and will oblige with anything from a brick through a window to a punch in the face and probably worse. He never likes to meet the punters face to face and works between three or four phone boxes.

Is there anything a little more light-hearted in his line of work? He has to think for some time. "Well, there was the man who wanted to get back at his wife's lover. Me and a mate trailed the guy and as he came out of this restaurant we got him with two paintball guns. The guy falls arse over tit and he's covered in yellow and green paint."

He then tells me about a few tried and trusted tricks that would make the Get Back boys envious. "You know you get cards in phone boxes, fresh young Filipino, water sports and all that. Well you get a dozen cards made up with the guy's phone number on and slap them back in the box. If your neighbours annoy you, phone up the electricity, gas board and pretend you live there. Say you're going away and ask to be cut off. Click. It's done."

You wonder why a character like Kenny would even bother to justify his sideline. But he does, frequently. Describing himself as "vaguely amoral", he says, "I've got a talent for rationalising things. A lot of people want someone to talk to. Nobody will listen to them. I suppose you could say I was a therapist." He drains his second pint and reflects on his service to the community. "I fulfil a function. Also, justice by proxy might stop them doing something much worse themselves"

`Inside Story: Dial V For Vengeance', BBC1, 10pm tonight.