Marc Bolan: The Final Word (2007 BBC documentary, over an hour long, on YouTube)
I want to recommend this as well as the Dandy In The Underworld documentary I recommended a couple of days ago ( https://democraticunderground.com/103469955 ).
The two documentaries cover much of the same ground, inevitably, but with enough differences to make both worth watching if you're a Bolan/T.Rex fan.
And The Final Word -- which is 20 minutes longer than Dandy In The Underworld -- has more contributions from Tony Visconti, talking about more of what was in his autobiography, which came out the same year, 2007, as this documentary. Again, I think that people interested in Bolan and Bowie's early careers would find that book interesting, since it has a lot more detail. Their careers would probably have been very different without their friendship and working relationship with Visconti, who was also a prodigy, with a professional music career that had started in his early teens.