AS CHELSEA threaten to throw away a winning lead in the Premiership, one metaphor occurs above all others: will they “do a Devon Loch”? It’s been 50 years since that horse became the most dramatic loser in racing — and probably even in sport — when inexplicably slithering to a halt on his belly 50 yards from Grand National victory.

The final frames of that 1956 coverage have been laboriously scrutinised but still fail to illuminate what happened to Devon Loch. He had jumped to the front at the last fence under Dick Francis and by the Elbow, the famous kink in the Aintree run-in, jockey Dave Dick on ESB had accepted his fate in second place.

The crowd roared, anticipating the first royal triumph in the National since Ambush II, owned by the then Prince of Wales, in 1900. The royal box, containing Devon Loch’s doting owner, the Queen Mother, along with the Queen and Princess Margaret, was seen in the first throes of celebration. But all cheers dried into audible gasps of shock.

In his autobiography, Francis would later blame that wall of sound for Devon Loch’s mysterious falter, enabling ESB to seize the advantage. More immediately, confused and dejected, his whip hurled to the ground in frustration, Francis had said: “The horse slipped . . . it’s as simple as that.”

Film evidence suggests Devon Loch, perhaps disorientated by the noise, went to jump an imaginary fence adjoining the very real water jump on the other side of the rail, realised his error and tried to correct himself but that momentum caused him to lose his footing and skid to a stop on his stomach.

In 1956, the attending racecourse vet speculated that an attack of cramp was to blame. Another contemporary theory was that a sudden small blood clot in the horse’s hind legs had caused temporary paralysis. My personal favourite was put forward when another national newspaper voted the incident “the worst mishap in the history of sport” — more intriguing than Mary Decker tripping over Zola Budd in the 1984 Olympics,; more unfortunate than Nigel Mansell’s tyre bursting in the 1986 Australian Grand Prix, losing him the drivers’ championship. The best theory has to be that Devon Loch was “destabilised by breaking wind violently after his girth was made too tight”. Brilliant. That’s definitely it. No doubt.

History notes several postscripts. That Devon Loch would undoubtedly have won in a record time. That the horse suffered no lasting damage, going on to win three more races before injuring a tendon. That his retirement prompted Francis’s own in 1957. That the Queen Mother was exemplarily gracious in defeat, first checking her horse, then comforting her dejected jockey and next heartily congratulating ESB’s winning connections. Francis recalls her saying, simply: “That’s racing, I suppose.”

Well, it is not so much typical of racing as typical of the Grand National. Every year, this race is a microcosm of the sport in just over nine minutes. One-offs happen all the time. Red Rum, Aldaniti, Foinavon — these names, among others, resonate in the history of Britain and sport.

The National looks like, sounds like and evolves like no other. It is all races in one. First, a 40-runner madcap jumble set off and incidents that in other races would hijack the commentary often barely register. Mêlées, refusals, jockeys flying through the air as their mounts unseat them — during the time it takes to complete the first circuit every year, the remarkable becomes commonplace.

It then morphs into an entirely different race on the second circuit, as it registers that some of the leading fancies have either taken against the big fences or had their chance hampered beyond redemption by the misfortunes of others.

Once the quick and the nimble negotiate Becher’s Brook for the second time, there are usually just a handful of runners with a serious chance of winning. Who gets going too quickly or too late? Who succumbs to a tired fall? Who finds to their cost that the 494 yards from the last fence to the winning post can be a merciless eternity?

Devon Loch may be history’s most famous loser, but his fate brought him immortality and laid down another strange chapter in the Grand National’s annals. His name lives on in our vernacular just as the race rejoices in its special place in the British consciousness. What will happen this year, when Saturday comes?

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• Graham Spiers