Did he win the lottery? Well so he says. Is he a born-again Christian? Praise the lord. Say hello to the most unbelievable man in Britain, then clear off before he has the shirt off your back.
The two men in the swanky West End art gallery are discussing the price of a nude. The woman in question lies supine on a bed of roses; buxom with pendulous breasts and a pot belly. She's going for pounds 11,000 but one of the men looks deeply unimpressed - big breasts or otherwise.

This man, a Mr Smith, boasts that he won five million in the lottery two months ago and has already bought a Rolls-Royce and a mansion in the country. Now he wants to get into art and impress his snooty neighbours, show them that he can get even with the toffs. The gallery owner nods sympathetically and smiles unctuously as Mr Smith gets into his stride, boasting shamelessly of his fortune.

Mr Unctuous thinks he's staring at a goldmine. Peering through his trendy round glasses, assumes the role of attentive host. "Well now," he says, ushering us to the back of the gallery, where cups of coffee and comfy seats are proffered. "Exactly how much were you thinking of spending?"

"Money's no problem - I'd spend pounds 50,000 on something I liked. Let's talk in football terms," says Mr Smith, happy with his working man's analogy. "What's a Manchester United gonna cost me? Or an Aston Villa? Put it this way, Who's the top player at the moment?"

He leans back in his chair, the centre of attention, engulfed in a huge donkey jacket, head almost shaved, with the odd tuft growing through at the back.

"The big sellers are Lucien Freud and David Hockney."

"Never heard of 'em," Mr Smith grins. For the first time this morning he's telling the truth.

Mr Smith is no lottery winner. He's a poseur. Dave Smith lives on a council estate in Colchester with his wife, Louise, 19, and their seven-month- old son. But he believes that he has single-handedly elevated the discreditable pursuit of hoaxing to an art form in itself.

Dave, 38, started winding people up from an early age. His definition of humour is one that involves relying on people's better nature to pull a fast one and then revelling in the last laugh. The coup de grace is to see his achievements recorded for posterity - preferably in newspapers or on television.

In his early teens, he and his mates would phone up each other's parents and pretend to be the council telling them that a group of Vietnamese boat people would be staying in their houses. "Obviously they'd go mad," laughs Dave. "It was so funny, We'd see them turning up at the council together to sort the problem out."

Dave, it seems, was the real joker of the family. "My brother's straight and serious. So's my mum - she doesn't like what I do. Basically, I suppose, I was the rebel of the family."

A few years later, Dave was borrowing his dad's car and pretending to be a mini-cab driver. The rest of his career is something he doesn't wish to dwell on or see in print. Suffice to say an "unfortunate accident" two years ago means he's lost the full use of one hand. So now he spends a great deal of his time at home, on the phone, planning the next stunt.

His greatest triumph came nearly two years ago, when he shot to tabloid fame on the front cover of The Sun ("I Won The Jagpot") for convincing car dealers that he'd just won the lottery. "Mr Scamalot", as The Sun called him, would swan around Essex from one local garage owner to another, test-driving Porsches, Rolls-Royces and Mercedes for four days at a time. Until he got rumbled - now no local car dealer will touch him. But due to Dave's voracious desire for an appreciative audience, being caught out is part of the pleasure. For him no publicity is bad publicity, even if it's 15 minutes of fame for just taking the mickey.

"The Spice Girls are no different to me - we both need publicity. And now people recognise me in Colchester," he says proudly. "They did a survey in our local paper with a photograph of me and Damon Albarn - 42 people knew who I was, and only 25 recognised him."

His ultimate goal is TV celebrity. "I honestly believe I've got the talent to make it somewhere in showbusiness. Surely you could sort me out with an agent?", he asks hopefully. "I'd love to end up with my own TV show. Give me the chance to have my own series and I'll make Jeremy Beadle look like an amateur."

Unlike Beadle, some of his jokes have gained him enemies in high places, not a good start for a career in showbiz, unless he wants to emulate the media saboteur Chris Morris. "Never 'eard of him," he says.

Kilroy-Silk, for one, isn't happy that he's been Dave Smithed. "They had a programme on debt-collecting and I said to them, 'Look, I'm a born- again Christian. I'm ashamed of my past - I used to be a leg breaker.' I said I'd only go in disguise, and they made me look like a young Charles Bronson."

Dave has also phoned up the powers-that-be at Cambridge football club and, after lengthy discussions, persuaded them that he wanted to buy them up. The club was bitterly disappointed when they found out who'd been up to his usual tricks. But Mr Hoaxer remains unapologetic. "I feel sympathy for the fans having to put up with a stuffy, snotty-nosed board of directors, thinking of themselves. But it never worries me that I've deceived people. If I worried I'd never do it at all."

Instead, after each scam Dave displays the sort of boyish glee rarely seen outside a playground conker fight. "Ye-e-e-s. We had him. He took it hook, line and sinker," he squeals as we leave the art gallery.

Two hours in Dave's company and you realise, warily, that the man has higher energy levels than a four-year-old who's pigged out on too many sweeties with E numbers. He talks non-stop and is in a state of constant animation.

" 'Ere mate, do you read The Sun?', he asks the taxi driver on our way to a Rolls-Royce showroom. "Did you see the front cover about the bloke who pretended to win the lottery?" The driver nods and Dave's ecstatic. "That was me. They've done a programme now - it's out this week." He beams, then he's off again boasting of another scam about standing up "two old tarts in a restaurant". By the 20th anecdote you begin to feel like a weary mother drained by her hyperactive charge.

By the time we reach the showroom, the one-gag joke has already worn a bit thin. This time the man in the suit seems a little less impressed - he barely blinks when Dave offers to bring around a pounds 20,000 cash deposit this afternoon. "We don't get many lottery winners," says the salesman rather sniffily. "They always seem to come from up north and stay there."

"Mmm. That wasn't so good," Dave admits afterwards, perhaps sensing that his audience of one is rapidly losing her sense of humour.

Ten minutes later we're in a burger chain, and Dave's burly form is starting to convulse; he slumps over his plate and spits out a half-digested mass of burger and cheese. Summoning the manager, he swiftly plants a bit of chewed matchstick in the middle of the gloop. "This is disgusting. There's something in my burger." he says outraged as other diners turn to stare. The manager, predictably, looks terribly concerned and offers a refund, which we refuse.

By this third episode, I begin to realise why spending time with Dave is such an unnerving experience. It's unusual to encounter someone with such an empathy by-pass for the people he puts out. His victims, you feel, are minor characters in the solipsistic Dave Smith plot. Which makes you wonder just how far he'd go for a front-page splash or a TV slot.

Dr Brian Thomas-Peter, a forensic psychologist who also appears on the programme, says of personalities like Dave's, 'It's breaching the value of human discourse - that we trust people will behave within acceptable boundaries. It reveals a disregard for convention at a fundamental level."

So Dave is a rebel after all. "He sounds more like the ultimate existentialist," Dr Thomas-Peter speculates. "He makes all values seem meaningless - as if they have no consequence or relevance. Kierkegaard and Satre would look ridiculous beside him." Perhaps a slight over-estimation of Dave's recent achievements.

Louise, his wife, has the real insight. "He's a bit obsessed about it all. Sometimes I find the jokes funny, but not 24 hours a day. I think he does it for the attention really, like a little boy." Which, if nothing else, may make him a natural for television. "Money's not the aim. I do really want to get somewhere, but this is my only route," says Dave, and for once you believe what he's sayingn

'The Lying Game', a four-part series, starts on BBC1 tomorrow at 10.20pm.