Today is the 220th St Leger, Britain's oldest classic horse race. Bookmakers expect pounds 8m to pounds 10m to be staked. Mark Holder will be tak ing a special interest. He's a professional gambler. Photograph by Marc Hill
In the betting ring at Exeter racecourse, the bookmakers are furiously compiling the odds for the fifth race - The Westrucks for Scania Handicap Hurdle over two miles and six furlongs. In the far corner of the ring Mark Holder starts fumbling about in a creased envelope stuffed full of pounds 50 notes. Immaculately dressed in a black cashmere jacket and grey trousers, he is a little under six foot with caramel brown skin and dark hair, flecked with grey. He approaches a bookmaker and places a thick slice of the folded fifties on a horse called Santella Boy. His voice doesn't falter. He could have been ordering a cup of tea.

Santella Boy's jockey gives the horse a beautiful ride. Holding him up round the final bend, he edges him through the field to lead at the last hurdle. He hits the front, goes two lengths clear, idles slightly, and is pushed out to win by a neck. Mark's expression doesn't waver. He ambles over to the bookmaker, who reluctantly scratches around in his satchel and pays him in crumpled notes. A bespectacled man in a turquoise windcheater pulls me aside: "You're with Mark today, are you?" he asks in a hushed voice. "What a guy, what a life. He's stuffed the bookies more times than I can mention. Winning for him is like taking candy from a baby. He's a legend."

Mark lives with his wife, Louise, and two-year-old son, James, in a detached house in a quiet middle-class suburb of Bristol. Inside everything is immaculate. Cream carpets, butter-yellow walls, a new three-piece suite and teak veneer tables. It could belong to a bank manager, doctor or insurance salesman. But it doesn't. It's owned by a man in his early thirties who makes a very comfortable living from backing racehorses.

Each day, Mark wakes up at 6.15am and pours over form books and the racing press, piecing together which horse is best to back. At midday, he'll set off for one of the West Country race meetings, place a sizeable bet on a horse that he believes is a value bet and return home to his wife and son for tea. It's a job like any other, but what goes on the table that evening is dependent on how half-a-ton of horseflesh performs over two miles and six furlongs on soft-going.

Mark's wife, Louise, is not fazed by her husband's occupation. But then that's not surprising. Soon, the Holders are moving out of their house to a pounds 250,000 five-bedroom house down the road. They are also swapping their Nissan 200SX turbo for a 4-litre Cherokee jeep.

Mark "works" from home. His office has the starched feel of a doctor's surgery. In front of him on his desk is a computer, containing the form of every horse that has run in Britain for the past five years. To his right are three monitors, one showing the day's racing, another the odds, and a third to record every national hunt race that day. Behind him is a shelf stacked with form books and ring-binders. He reckons he spends around eight hours a day studying the form and watching past races. "If I was a double-glazing salesman and I put the amount of hours in that I put into gambling, I'd sell a lot of windows," he says.

A few years ago, Mark and his brother Paul staked pounds 14,000 on the result of a photo-finish at Sandown racecourse. They lost the lot. It was hard to come to terms with but gambling to someone like Mark is an incurable addiction. "Gambling is in my blood," he says. "My whole family have always been into it. My grandfather, who was a farmer, once lost the milk cheque betting on a tug-of-war pull - it was as bad as that."

Mr Holder senior was a farmer who turned a moderately successful dairy farm into a thriving racing stables. Mark left school at 16 to work in the yard as an assistant trainer, but it was the gambling side of the job that really appealed to him. Eight years ago, he "made the hardest decision of my life" and left his father's yard to become a professional backer full-time.

He says he earns anything from pounds 25,000 to pounds 55,000 a year from gambling, which is somewhat disappointing. Most people have visions of professional backers gambling the sort of figures most of us associate with 25-year mortgages and winning cash of lottery-jackpot proportions, quaffing champagne in racecourse bars across the country. "Not a bit of it," says Mark, "to gamble professionally you have to be incredibly disciplined and methodical. I only bet if I'm convinced a horse is going to win, and most importantly it has to be at the right price. I only bet on one horse a day and sometimes may not bet at all. I bet purely for profitability."

This disciplined approach was brought about by years of reckless abandon. "I've had a few stupid bets in the past, which I regret, and I had my fair share of fast cars, boozing and night clubs. It was very up and down in those days but I was still learning. When I got married I realised I had to be a bit more responsible."

This stability came in the form of a tipping service, which Mark now runs with Paul from a shop in Portishead, Bristol. Paul looks after the administration, Mark and his partner Alan Potts, another pro-gambler, provide the tips. Punters pay pounds 69 a month and in return get a secret phone number which they dial each day to hear the two experts' selections. Over the past five years all three men have built the business into one of the most successful and profitable tipping services in the country.

At Exeter racecourse, Mark is known in racing language as a "face", which means he is someone in the know. He is the man who the bookmakers fear and the punters worship. As he prowls around the bookies' ring he nods to trainers, jockeys come up and shake his hand, bookmakers mumble nervous greetings. Others hang around like vultures, too scared to converse, but keeping an ear cocked for any words of wisdom he might accidentally spill into their lap.

He stands by the saddling enclosure and sizes up the horses for the next race. Carefully marking his racecard, he spills out jargon: Some horses are "poor walkers" others are "backward" in condition. As they run down to the post, he sighs: "Look at that knee action, there's no way that horse will go on this ground. It needs soft, that's definitely one to remember for next time."

Loitering by the parade ring are three other professional backers - friends, but essentially the enemy. Mark joins them and they all chat amicably, moaning about the last race, discussing the options for the next without actually committing themselves to which horse they are going to back. The unwritten rule to be a member of this clan is to keep your selection quiet. Otherwise everyone backs the same horse and the price depletes rapidly. Towering above the other three is Eddie "the shoe" Freemantle, a former journalist for the Sporting Life, who quit Fleet Street to back horses for a living. He wears trainers. Big ones. Size 13 at least.

A nervous-looking man with receding red hair and a face riddled with boils sidles up to the group. Wearing a musty tweed jacket, nylon trousers and hush puppies he fidgets like a crazed animal with his brown leather binocular case. One gets the impression he's lost a fair whack. "Where's the value in National Hunt racing these days?" he moans, "I mean how could you have backed the winner of the last, eh?" His left eye begins to twitch violently: "And look at the next race. I wouldn't touch those nags with a barge pole." After a while it becomes apparent that the man with the boils is one of life's bores. Mark and "the shoe" slowly edge away from him.

At the end of the day's racing, Mark is looking fairly pleased with himself. He's won a fair sum and tipped the winner of one race on his tipping service. He says he won't be able to be a pro-gambler for ever ("If I did I'd end up in a padded cell one day with someone passing me my toast under the door") but admits that it beats the life of a double-glazing salesman. As he makes his way back to the car park, he takes the brown envelope out again and has another quick rummage. His face has a look of pure ecstasy. "Um, not a bad day," he says with a smile, "not a bad day at all."

Serena Mackesy is on holiday