That's right, Warwick, I said boxer shorts only: DJ Caesar is big on practical jokes. If you get up early enough, you may catch some of his listeners doing some pretty funny things, as John Godfrey witnessed

WARWICK from Stepney Green is going to cycle in his underpants to the middle of London Bridge and busk at five o'clock in the morning, playing his flute. 'I can't believe I'm doing this,' Warwick confesses, and to be honest, neither does anybody in the Kiss FM studio. Last week Greg from Isleworth drove naked in his Escort RS Turbo to the studios in north London with a cooked breakfast for DJ Caesar.

If you tune in to Caesar's The Party Line phone-in show on any weekday between 4am and 7am, you can hardly believe your ears. Here are people who are crazy enough to shave their heads, go to work dressed only in boxer shorts with a forefinger up their nose, walk three miles at 5am, wearing only a pair of shoes and a placard declaring 'We Want Sex in Prison'. And all for the promise of a leather jacket with Kiss FM embossed on the back.

Promise, because sometimes, even if the lunatics do what they're asked, they don't even win the prize.

Caesar is a 'shock jock', a DJ who generates controversy. When he says on air that 'it is wrong for women who go out for a night of lust to have abortions', and that 'out and out perverts should have their nuts chopped off and used as experiments instead of rabbits', he listens to the umbrage he arouses rather than cutting people off. 'I never put the phone down,' he declares proudly.

He says he never prepares for his programmes - 'it all depends on the mood of the people who are listening.'

Tonight the mood is distinctly solemn. Caesar has a serious side: shows earlier in the week included two calls from listeners contemplating suicide, and yesterday's was dominated by a call from Sonia, a single mother whose marital difficulties have seized the audience's imagination.

The Sonia soap opera continues for nearly two hours, accusations and counter-accusations coming fast and furious. And then Warwick from Stepney Green rings in. The studio breathes an audible sigh of relief. Warwick wants to do a forfeit. Tonight a Sega Megadrive computer game console and a television are on offer, and Warwick, a music technology student, is going for the television. Caesar always decides the particular forfeit on the basis of the caller's occupation.

The photographer and I leave the studio wondering if Warwick even exists. When you listen to forfeits on The Party Line for the first time, your main reaction is simply disbelief. These people can't be real. Which is exactly how a wind-up works. Regular listeners occasionally phone in pretending to be fictional characters, trying to catch Caesar out. People who attempt forfeits phone in claiming their prizes, only to be unable to refute the response: 'I don't believe you.' If you are an invisible voice on the other end of the phone, claiming that you are having a bath in the fountain at Trafalgar Square at 6am, how does Caesar know you're not in your own bath at home with an accomplice who's pretending to be a passing policeman? It becomes call my bluff, a game of wits. Tonight I'm pretty sure that Warwick has lost his.

He's there, cycling along London Bridge, his exposed flesh chafed crimson by the sub-zero temperatures. He didn't cheat, he says, because he was worried that the cab drivers who, Caesar tells his listeners, act as referees would spot him, and then he would be disqualified.

Suffice to say, the only referees this early morning are from the Independent. Early commuters rub their eyes to make sure they're not half-asleep, one actually puts some money in Warwick's box, and a passing Japanese banker is shanghaied as proof that Warwick did what nobody believed he would.

Warwick's problem is that Caesar still doesn't believe him, suggesting that one of us does a very good Japanese accent. Caesar capitulates, pausing before the punchline: 'Warwick, I haven't got a telly]'

For 5.30am, Warwick took it very well, managing a smile when he called Caesar a bastard.

Listeners in Sussex, Kent, Surrey and Essex will know Caesar already, where he had a show on Invicta Radio and then Essex Radio for seven years. When he parted company with Invicta, 300 people demonstrated outside the station. It is the loyalty of Caesar's listeners that enables him to get away with the sort of practical jokes that Jeremy Beadle can only dream about. Caesar's listeners, it seems, would literally do anything for him.

He says: 'Loyalty is a very hard factor to come by in radio, and I find that if people like you, and know you're a genuine person, they'll follow you anywhere. I've been doing the same show for seven years, but now I'm on Kiss, which is known as a 'radical radio' station, it's a bit stronger.'

The cream cakes and sausage rolls addressed to 'Caesar and the Gang' that a local bakery delivers every morning are typical of his listeners' mentality. To listen to him is to belong to a gang.

It's 6.30am on King's Cross station. After taking pity on Warwick, we offer to give him a lift back to Kiss to pick up his 'booby prize'. The Kiss jacket he eventually gets will prove useful: he's still half-naked. But first there's Roy, the London Underground supervisor who rang in wanting to win the Sega Megadrive. His forfeit was to wash a British Rail train by hand, including the interior, and his grip on his chamois leather has a pyretic quality that is matched by his smile.

He listens to the show every night and says he is hooked, addicted to The Party Line. Tonight's show was not the jeu d'esprit he expected, but he describes the saga of Sonia and Raymond's marital problems as 'really good, giving both sides of the story'. And anyway, he adds, don't tell Caesar.

At 7am, we arrive back at Kiss with Roy and Warwick. Roy walks away with an assorted armful of Kiss paraphernalia. Caesar didn't believe him. Even if Roy had managed to clean the InterCity carriage, he wouldn't have collected his prize anyway. Caesar gave the Sega Megadrive away to Sonia's 11-year-old son, Stephan.

(Photograph omitted)