This documentary is probably, as they say, one for the heads – for connoisseurs who appreciate a great musician who was never a star. But the flaw of this film, admirably detailed and celebratory though it is, lies for me in the fact that it never pauses to wonder why exactly he was never a star, and what that precisely means. Does star quality consist, in some mysterious way, in a lack of formal musicianship?

Nicky Hopkins was a superbly accomplished pianist who played on records by the Who, the Kinks, Jefferson Airplane, the Rolling Stones and the Beatles (plus solo albums by all four ex-Beatles) and many more. His brilliant work meant he was admired and even hero-worshipped by musicians and producers on both sides of the Atlantic. He was a classically trained Englishman (like Elton John at about the same time, he studied at the Royal Academy of Music) yet sounded as if he learned piano in the Mississippi Delta. And all this was while Hopkins was dealing with serious ill-health; he had Crohn’s disease and later issues with drink and drugs. The latter were at least partly due to a need to dull the pain, in order to keep up with tough recording and touring schedules; it all contributed to his heartbreakingly early death at the age of just 50.

So why is this amazing figure not world famous? Partly it was his refreshingly ego-less approach (although he had no false modesty, and only half-jokingly called himself the reincarnation of Chopin). His humility made him loved, and made him a great collaborator. The session musician’s life – free of the star’s burden of leadership – unlocked his creativity. He was also no snob; to the bemusement of some interviewed here, he worked on David Soul’s solo album. Yet real stars are perhaps also immodest and obnoxious in ways alien to Hopkins: they may not have his technique and his artistry, but have some raw originality and inspiration that Hopkins arguably didn’t. Art isn’t fair and life isn’t fair.

In fact, Hopkins did release some solo albums, which are respectfully namechecked here, and yet nobody talks about the unmentionable subject of why they didn’t take off – perhaps because of a need not to sound disrespectful. I would have liked to hear some frank discussion of this, but there are still valuable insights here.

• The Session Man is in UK cinemas from 21 November.