But the house now looks set to smash its own record in October, when it offers what is undoubtedly the most important Brough in the world – aside, that is, from the marque's 'holy grail', the one on which T.E. Lawrence famously died after crashing near his Dorset home in 1935.

The machine being sold by H&H is the celebrated Old Bill, the personal 1922 racing SS80 of company founder George Brough. How his fellow competitors laughed at the relentless pit-lane pampering of the bike they unkindly named 'Spit and Polish' – until Brough won the experts' scratch race outright and set a new lap speed of 100mph.

Brough officially named the bike Old Bill after the famous wartime cartoon character of artist Bruce Bairnsfather and went on to win 51 of his next 52 sprint races, too – only being denied victory in the last, after he fell off. The bike, of course, dutifully kept going and crossed the finish line in the fastest time of the day, but Brough ended up in hospital and Old Bill had to be sold to fund the staff wages.

While in storage during WWII, it was damaged by a falling bath tub and went unused until being acquired in the '50s by Titch Allen, legendary founder of the Vintage Motorcycle Club. With Brough's assistance, Allen restored the bike to its 1923 specification before giving it to his son, Roger, in 1988.

His wife, Sue, subsequently inherited the unique machine following Roger's death during a motorcycle race on the Isle of Man in 1992.

Old Bill came to H&H through its motorcycle consultant, the classic bike guru George Beale, who says its combination of significance and provenance is 'unrivalled'.

Although H&H’s £286,000 sale does represent a world record at auction, the first factory test machine was rumoured to have been on the market for £500,000. Only time will tell whether or not Old Bill is worth more than that... but it certainly ought to exceed the 1923 SS80 list price of £150.

Old Bill goes on sale at the Imperial War Museum, Duxford, on October 4.

Photos: H&H Classics