I am assuming that this film I found on Youtube is the 1968 BBC documentary. I've been wanting to see it for years, and now that I have, I'm very disappointed. For being a BBC production it has all the hallmarks of a student film, made by snarky college kids trying to be clever and 'controversial'.
It is recorded elsewhere that Tolken did not enjoy being filmed. Also, of the "miles and miles of film" shot of Tolkien, very little made the final cut. Instead we see a jumble of disjointed scenes of Oxford and various students voicing their opinions.
The footage we do see of Tolkien I think is pretty good; at least the scenes where he is allowed to talk about his work. I especially like the bit where he visits a couple of favorite trees.
I also like the girl we see at the beginning, and interspersed throughout, as she goes on and on and on, giving us the plot of the Lord of the Rings. She seems a nice girl, and a true Tolkien geek.
I've noticed that when reading or reciting Tolkien speaks in a very clear and strong voice. Only in conversation does he lapse into the rush of muttered half-formed words and sentences tumbling over each other.