It sounds like something from the pen of Sir Arthur Conan Doyle: the curious case of the royal family, the Italian confectioner and one of the most dramatic moments in Olympic history. A mystery that has finally been solved, thanks to a retired civil servant and King Charles.

Like many classic detective stories, it starts with a body on the floor. One hundred and 16 years ago on Wednesday, the 22-year-old Italian Dorando Pietri was leading the 1908 London Olympic marathon only to collapse yards from the finish. Five times he fell. Five times he was helped up. To the roars of nearly 100,000 people at the White City Stadium, he staggered over the line in first place.

Related: Paris 2024 explained: all you need to know about the Olympics but were afraid to ask

Watching on, transfixed, was Conan Doyle. “Amid stooping figures and grasping hands I caught a glimpse of the haggard, yellow face, the glazed, expressionless eyes, the long, black hair streaked across the bow,” he wrote. “Surely he is done now? There is a groan as he falls once more, and a cheer as he staggers again to his feet. It is horrible, and yet fascinating.”

It was all to no avail. Pietri was disqualified for receiving “outside assistance”, with the American Johnny Hayes declared the winner. However, Pietri’s exploits made him an overnight celebrity with Queen Alexandra giving him a cup for his valour and Conan Doyle raising £300 (about £40,000 in today’s money) on his behalf.

That hot London day was not only dramatic, but historic: it marked the first time the Olympic marathon was run over 26 miles and 385 yards, which for more than a century has been the event’s official distance. Until then, it had been loosely around 25 miles.

One hundred years later, there was a shock twist. The former Daily Telegraph editor John Bryant revealed that the 1908 course had been remeasured using “maps, contemporary photographs and 21st century methods” and was found to be 174 yards (159 metres) short. In other words, the famous distance was based on a mistake.

The measurer? No less a figure than John Disley, the co-founder of the modern-day London Marathon. According to Disley, the discrepancy may have been caused by the starting position at Windsor Castle being switched at the last minute from the Long Walk to the Eastern Terrace. When Joe Neanor, a retired civil servant, read the story he recoiled in shock. “As a marathon runner I was devastated,” he said. “I thought: ‘We are running this distance and it’s a lie.’”

So began a four-year odyssey to prove Disley wrong. The area of dispute centred on the first 700 yards inside Windsor Castle. The problem was no one knew where the start point was with the official report vaguely stating “the race starts from Windsor Castle, near the East Terrace, 700 yards from Queen Victoria’s statute”.

It appeared a thankless task. But Neanor was rewarded for countless hours trawling through newspaper and photographic archives. It came in the form of a panoramic image in the Illustrated London News, showing the castle, the runners at the start and the Crown Prince of Sweden marking the line.

“It’s the only photograph that brings those two elements together in a single image,” said Neanor. “And it only exists because of the two children at the bottom of it, Princess Mary and the future George VI, the grandfather of our King Charles. The photographer clearly wanted something beyond a bunch of runners so took an image with the royal children.”

Now he knew the starting point, Neanor asked Hugh Jones, who won the London Marathon in 1989 and measures marathon courses distances for a living, to help out. But he faced a further problem: Windsor Castle is the property of the crown so the two men could not just rock up with a tape measure.

He wrote to King Charles, asking for permission. To his delight, the monarch said yes. A Windsor Castle spokesperson said: “Given the private area that we were working in, The King gave His personal approval for the exercise to take place and expressed His interest in the outcome.”

On a rainy day this year, Neanor finally put his theory to the test, helped by Jones’s trusty measuring bicycle and the superintendent of the castle, Col Duncan Dewar. Jones started by measuring Queen Victoria’s statue through to the Wicket Gate, making chalk marks to define each section of the course, before noting the distance. The first part: 285.5 yards. The remaining section of the East Terrace wall: another 86.5 yards.

Soon Jones had an answer: the first 700 yards of the 1908 marathon had been accurately measured. Neanor had been right.

There was a handshake between Jones and Neanor and a photograph of the moment for prosperity. But that might not yet be the end of the matter as Windsor Castle is considering acknowledging Neanor’s detective work. “We are examining the possibility of marking the position of the start for future reference,” a spokesperson said. “But nothing has been agreed at this time.”

Whatever happens, Neanor has the satisfaction of having proved that the London 1908 event was not just the genesis of the modern‑day marathon as a spectacle, but a race that really does last 42,195 painful metres.