A Rolling Stones recording from 1964 that is so rare even Sir Mick Jagger has never heard it is the prized possession of an Essex mini-cab driver, who has kept the tapes locked away since his brush with the legendary rockers.
Now John Mackswith's demo version of the band's "As Tears Go By" is set for its first public airing as the legendary rockers scour the archives for a major re-release programme.
Mr Mackswith, now 63, was a slightly awed 17-year-old tape operator at the Advision recording studio in New Bond Street when fast-rising blues revivalists the Rolling Stones arrived to lay down some demo recordings.
The band sped through "As Tears Go By", one of the first songs that Jagger and guitarist Keith Richards wrote together. Mr Mackswith liked the song but was disappointed with the band's ramshackle performance.
When the band and studio staff left, he secretly "remixed" the tapes, creating his own improved version of a song that would become a hit for Marianne Faithfull and later the Stones.
The only copy of Mr Mackswith's exclusive version currently lies, carefully stored in 10.5in magnetic tape reels, in a suburban house in Chigwell, Essex. It hasn't been heard for nearly 50 years. But the driver is willing to let the band assess its potential as the Stones seek out rarities as part of a reissue programme of their classic back catalogue.
"When everyone left the studio in the evening we were let loose on the equipment," he said. "I used spare tape to make a couple of mixes of my own. It was probably as bad as their performance; I used too much reverb. I took them home as mementoes."
A spokesman for the Stones said: "This could be a very interesting discovery." The recording came to light when David Bowie's UK representative booked an Addison Lee account car to a gig and got talking to Mr Mackswith.
Lost and found: Treasured tapes
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Two episodes of Dad's Army were found in 2001 in a garden shed where they had been since being retrieved from a skip outside Elstree Studios.