Like so many perfect ideas, the appeal of The Gambler is based on blinding simplicity: giving a guy £12,000 to blow on gambling and filming his inevitable misery is an appetising prospect. But to choose someone who all but came apart after undergoing an identical experience is both fiendish and brilliant. Jonathan Rendall, a novelist and charmer, is the star of Channel 4's three-part series.

For some reason, the first programme did not fully highlight the fact that Rendall was offered the exact same amount of cash by a publisher a few years back on the understanding that he would write a novel around his experiences. He duly flittered away what he regarded as a fortune on a series of heartbreaking bets, and after penning his novel was hospitalised with an obscure condition labelled the "myclonic jerks".

No sooner had he pulled himself together than Channel 4 called by waving a similar wad, asking merely that they be allowed to film his hapless descent into red-eyed, odds-on despair once again. And Rendall, like all true addicts, couldn't say no.

The series is so promising because of the subject himself. Rendall is a refreshingly antitweed winner of the Somerset Maugham award for young writers, chain-smoking and effing his way through England's more depressing racing and gambling haunts in order to re-acclimatise himself. Horses are his passion, as he grew up near Epsom and, at the outset of this documentary, was living in a shambling farmhouse near Newmarket. The big advantage of this, he said, was that he could scrutinise "the going" from his back garden, which he did by dropping a brick (which he estimated would have the same impact as the horse's hoof) onto the turf. That example of a layman's haphazard science would prove typical of his reasoning at the racetrack or at ringside.

Graham Sharpe, of William Hill, who first encountered Rendall on his previous excursion into the numbers world, allowed that the writer was "an exceedingly optimistic gambler". He revealed this in a tone suggesting that such optimism was not necessarily a good thing.

"It's not the money that counts; it's getting the omens right, getting the light to shine on you," declared Rendall at the outset, embarking on a little tour of the gambler's playground to warm himself up. So many potentially ordinary sequences are rendered amusing by Rendall's willingness to ham it up as a modern day Cincinnati Kid. He appears to adopt the same attitude towards life as he does to his newly acquired bundle: that it is there to be blown, that you are going to lose it sooner or later.

So when he swaggers into a sidestreet newsagent and asks for 100 scratch cards, he is taken aback when the shopkeeper doesn't swoon in astonishment at his casual abandon. He is informed that people regularly squander (their own!) cash on 100, 200 cards a week in this anonymous little shop.

"I came in here with the pretensions of a high-roller," he sighs in dismay. And this is the key to The Gambler. Rendall loves a flutter, gambling fascinates him and he knows a fair bit about it. But even he is unprepared by the excesses and stories of the characters he meets along the way.

"You won £29,000," he exclaims, in conversation with a pipe-smoking Irish gent at Cheltenham who seems less than awed by his own earnings.

Rendall's awe and appreciation of Cheltenham is easily apparent, but it is his visits to forgotten meets like Lingfield that are more appealing. "When you go into this," he says of gambling, "it's another world entirely."

And how hellish are the environments such as Lingfield, with their cast of misfits and grey afternoon races and all that lost money. For behind the frivolity of this venture, the friendly ease with which Rendall sets about squandering his £12,000, there is a hard edge to what we see. Without taking any moral stance, the implication that gambling is a game that ruins lives is never far from the surface.

The only sign that he might be in line for another hospital spell was in the final scene, when he placed £1,000 at 10 to 11 on Flagship Uberalles at Cheltenham. Despite his fraternising with bigtime hustlers, Rendall is chiefly a writer living an uncertain financial existence, and the decadence, the magnitude of this bet, unnerved him.

Channel 4 showed the race by filming his reactions during it. It's hard to believe there will be a better television moment all year. His horse, his beauty, sucks him in, promising so much, stumbles on the last, recovers and rallies only to lose at the end. Rendall lives this through a flurry of curses, grimaces, jumps, bends, turns and, quite probably, myclonic jerks.

"My legs are f***ing shaking," he says afterwards. "That's really done me in." And he truly is devastated, so overcome that he has to leave to be alone.

But by the end of the hour's programme, Rendall is down to a little over £8,000. His next two films remind us that gambling and smoking are the last remaining legal global opiates.

There is already a sense our man is taking the fast path to Brokesville. It's a safe bet the story will only get better.