A several-hundred-strong mob storming a police station seems more redolent of the Wild West than commuter-belt Surrey, but that’s what happened on Wednesday, June 18th, 1919, when about 400 Canadian soldiers led a charge on Epsom police station. Their aim was to spring two of their colleagues arrested after a fight in The Rifleman, a local pub.

The two prisoners had been arrested after brawling in the pub on Derby Day. Twenty of their comrades then gathered outside the town jail to demonstrate, sending word back to other soldiers, at Woodcote Camp on Epsom Downs, to come and join them.

Bored, homesick and resentful at the slowness of their repatriation after the First World War was over, the 400 “reinforcements” charged through Epsom, breaking windows and smashing property in a trail of destruction more than a mile and a half long.

They stormed the police station, defended only by 16 police officers armed with wooden truncheons. During the fighting 11 policemen were injured, and Sergeant Thomas Green, 51, was fatally wounded when he was hit on the head with an iron bar. He died in Epsom Hospital next day without regaining consciousness.

Initially, detectives believed that they had isolated the riot ringleader and Sergeant Green’s killer when they arrested Allan McMaster, 30, a former blacksmith. But the Canadian camp swiftly closed ranks and witnesses refused to say anything. Even so, McMaster and four other soldiers were put on trial at Guildford in July, accused of rioting and manslaughter.

The prosecution then presented a case which was so bland and anodyne that the four were swiftly acquitted of manslaughter, found guilty only of rioting, and sentenced to 12 months’ imprisonment. Within weeks they were pardoned by the Prince of Wales and sent home.

In 1929 McMaster confessed to the “murder” in Canada and was detained by the police, but released after Scotland Yard said he had been dealt with by the Guildford court in relation to Sergeant Green’s death. This left the case officially unsolved.

So why the unseemly haste in drawing a line under the murder and the rioting? In the 1990s a former Scotland Yard officer, Edward Shortland, claimed that the killer was deliberately allowed to escape the hangman for fear that his execution would destabilise a royal visit to North America.

“Only orders from the very top could possibly have enabled such complicity,” he says. “As the Prince of Wales was about to set off on his visit to Canada, long prison sentences and perhaps several hangings of Canadian soldiers would have been a public relations disaster.”

Those Mr. Shortland believes were in on the plot to damp down the affair so as to make things easier for the royal tour were Prime Minister Lloyd George, Winston Churchill, the prosecution and even the trial judge.

The farce was completed when the police officer in charge of the case was given an official commendation, even though he never “caught” the killer.