He proved the inspiration for the Beatles’ mop tops and won a Grammy for designing their Revolver album cover, but on a balmy summer Saturday morning in 1961, Klaus Voormann nearly became the Fifth Beatle.

Their original bassist, Stuart Sutcliffe, had just played his last gig in Hamburg’s Top Ten Club, opting to leave the pre-fame band and remain in Germany with his girlfriend, photographer Astrid Kirchherr, to become an artist. In a chemically altered haze, and already part of the inner circle of Paul McCartney and company, Voormann thought there might be a vacancy and staked his claim.

“Well, it’s a silly story, because the band then was not important,” Voormann laughs today. “Nobody knew who those boys were. But I thought they were great and would have loved to play with them. We were all outside and sat on a heap of wood, and we were all a little stoned. Just for fun, I said, ‘Well, maybe I could play.’ And then John Lennon replied: ‘Ah, Paul’s already bought a bass.’”

Days earlier, McCartney had purchased the German-made violin-shaped Höfner 500/1 for £30 (around £600 today), which he played on the Beatles’ early hits, including Love Me Do and She Loves You. However, in 1972, the instrument mysteriously vanished – purloined from the back of a van in the Notting Hill area of London. Now, a charming documentary, McCartney: The Hunt for the Lost Bass, unpacks the twist-filled 51-year quest to reunite Macca with the “Holy Grail of rock ’n’ roll” (as his Höfner has been dubbed).