He has been out working a horse on the gallops. "Not a sinner about," he says, with wry satisfaction. "Sunday morning, they're all in bed." But the wind has exacerbated his conjunctivitis. There's something wrong with his ears, too – they're stuffed with cotton wool. Barney Curley moves slowly, nowadays, can't walk very far. Last year he lay in hospital for three months. "Lucky to get out. It was evens each of two, live or die." He's 70. And three weeks ago he pulled off one of the most extravagant gambles in the long history of the Turf.

"Nobody will ever win as much on horse racing, this century," he pronounces, in his measured Co Fermanagh tones. Quite how much, he will not say, though industry estimates of £1m sound hopelessly conservative.

One of the men who helped manage a project of rococo complexity, joining us in Curley's sitting room, suggests that it was first discussed before some of the horses involved were even born. Come the day – a humdrum Monday 10 May – four were linked in a series of trebles and accumulators. Three are trained by Curley himself, in probably the smallest stable in Newmarket. The fourth he had sold in 2008 to Chris Grant, a trainer on Teesside.

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Agapanthus won at Brighton; then Savaronola did the business at Wolverhampton. But Curley's third runner, Sommersturm, was beaten later on the card. That left Grant's horse, Jeu De Roseau, who made his first appearance in 742 days to win at Towcester's evening meeting.

Had Sommersturm completed the job, the bookmakers would all have reached their various maximum payouts – an aggregate Curley reckons at over £20m. But it has been hard enough getting them to pay out, as it is. "I'm pleased the other one didn't win," he insists. "If these fellows can't pay three, what chance would we have with four?"

Previously, Curley was most celebrated for Yellow Sam, who won at a country track in Ireland in 1975. There was only one telephone at Bellewstown, and Curley had a friend act out a prolonged call to a fictional dying aunt, so blocking desperate attempts by off-course bookmakers to cut Yellow Sam's starting price. His winnings have been computed as the equivalent of €1.7m (£1.4m) today.

Such a ruse, of course, could not be entertained since the advent of mobile phones. "People were telling me that our day had gone," Curley says. "You know, punters I knew over the years. It's finished, they said, over. I never thought like that. Because bookmakers are always trying something new, to rob punters, to get them to bite. That's what beats them. The greed."

And that's what spurs Curley. He doesn't need the dough. Since the loss of his teenage son, Charlie, in a car accident in 1995, his chief purpose in life has been a charity he set up in Zambia. In his youth, Curley studied to become a Jesuit. For all the picaresque and iconoclastic flourishes of his life since, he is respected by some of the most eminent horsemen of his era.

Sheikh Mohammed once facilitated a donation of £2.5m to his charity. Trainers with 20 times as many horses in their care consult his opinion. When they arrived as teenagers, from Italy and Ireland respectively, he was mentor to subsequent champion jockeys in Frankie Dettori and Jamie Spencer. His latest protégé, Tom Queally, will be joining them in the Investec Derby on Saturday.

So why persevere with the precarious adventures that redeemed him from the penury of younger days? "It's not for the money," Curley says. "It's for the buzz. Beat the system, you know, beat those bookmakers, those smart-arses. You go into a betting shop and see them robbing these poor fellows, with these gaming machines. They're as addictive as crack cocaine. You see them coming back to the counter with their credit cards, for another tenner. Of course the great thing about those machines is that number nine won't go to even money and win five lengths."

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Equally, he remembers sitting in hospital and reading about footballers on £100,000 a week. "I know they're the best at what they do," he says. "But here's me, the best at what I do. And every year, when I came to a certain figure, I said: 'That's enough'. But now I thought: 'I've been underpaying myself the last 15 years. My job's a lot harder. It's about time I caught up with these fellows.'"

Granted that it all pays off, the planning almost seems its own reward. Curley invokes a draughts board. "You'd change the pieces hundreds of times," he says. "Put horse A there, and have horse B in here. But this one's not going well, that one's lame. And we're operating with very few horses, you know. Take horse B out. A week later, back in again. It's not easy. Horses are so unpredictable."

The one that excited most curiosity is probably Jeu De Roseau, but Curley has nothing to hide. Grant is a friend of Andrew Stringer, Curley's assistant. "And I've sold him numerous horses over the years," he says. "If there are five gentlemen in racing, Chris Grant is one of them. A decent, honest, hard-working fella. We were at this sale, and I said to him: 'You should buy this one, there could be a turn in him. He's been sick, given us nothing but trouble. But he did show a bit of form, back in Ireland.' A thousand quid. Can't go wrong."

A while ago, Grant telephoned. The horse had begun to thrive. Curley was sceptical. What would Grant have, to work him with? But he was insistent. "The horse had a very bad virus when he was here, looked terrible," Curley remembers. "And he was saying he was looking well now, that he's turned a corner."

Grant was thinking of running him at Towcester. "That's funny," Curley replied. "I've been trying to find a horse to run in the seller on the card." He had been ringing round, looking to fill another barrel in the bet. But nobody had come up with the right horse. In the event, Jeu De Roseau enabled Curley to switch his sights to a handicap instead.

But the real miracle was for three of his own horses – he only has 11 – to peak together. Their own reformation was mental rather than physical. "Agapanthus turned nasty last year," he says. "We rested him, he loved his day out hurdling, just began to shine. And we did the same with [Savaronola], he was a right nasty piece of goods when he came. One day at Southwell he kicked the place down. Now he's as quiet as a lamb. The horses here have the best time of any stable in the world."

A strategy was gradually refined. A network of agents picked, tested, discarded or trusted. Bets synchronised. "Fifty pounds in the wrong place," he says. "That's all it would have needed. And most people are untrustworthy, when it comes to money."

But much else remains beyond control. There were evidently other days, other horses. It was like postponing the Normandy landings for bad weather. When Grant's horse entered the equation, 22 other entries had to be scratched just to get a run. "Same as someone robbing a bank," Curley says. "The next thing, hasn't been seen for years, but there's a police car parked over the street. You can't plan for things like that. And then, on the Saturday morning, one of them was dead lame. The vets were here, the blacksmiths were here. There was no shouting, no roaring. Quarter to ten, I went up to Mass. If it's going to come, it's going to come. And the next couple of days it got better, and he was just sound to race. In normal circumstances, I'd have done nothing with him for a week."

Another medical drama, for Curley himself, saw him detained in hospital until Monday afternoon. "I get out of all this carry-on at 3.40, so I'm just back in the house to watch Agapanthus at 4.10," he says. "But it was nothing to do with stress or anything. Just my blood was wrong. I'd be watching those races like I'm sitting here now, smoking. My heart would be..." He holds out an impassive hand. "You know, we'd done all we can, that's it."

Now he winds up the flash young jockeys, asking which is the best Mercedes on the road? But all he really wants is to get back to Zambia. September at the earliest, according to his doctors. Direct Aid For Africa has built a school for 1,600 in Zambia. "In racing, people always want to get on your arm," he reflects. "The people giving their lives out there, they don't want anything off you. And once you've been out, it draws you back – those children with their big brown eyes looking up at you, with nothing to eat."

His wife, Maureen, reproaches him for squandering his gifts on horses. "She says I should have been managing director of Tesco, something like that." He pauses, shrugs. "You see, I believe peace of mind is a great thing to have. And I've wonderful peace of mind."

After reading about the coup, Dettori telephoned Curley. "I hear you've had a touch," he said. "I'm pleased. Because the news on the street was that you were losing it."

They had begun to forget about Barney Curley. And now, suddenly, he has left an immortal footprint on the Turf. "It's something I don't think will ever be done again," he says. It will be a good while, however, before any bookmaker grows at all complacent in that assumption.

To learn more about Curley's charity, go to www.dafa.co.uk

Barney Curley

Age 70

Curley managed pop band Frankie McBride and The Polka Dots, the first Irish band in the British Top 20.

£1.4m

Adjusted value of Curley's famous betting coup with Yellow Sam in 1975. Some disgruntled bookmakers paid his winnings in single notes, filling 108 bags.