It's from the last five years and was aired on NBC. I believe it's called "Time Again" or "Time and Again".
It chronicles the OT by showing clips of the films along with interviews with all the principal actors and performers from 1977 right up to the late 1990s. Most of which I have NEVER seen before.
This was recorded on an NTSC tape by my sister in the US. I had a PAL VCR in the UK and until now I've been unable to watch it. Unfortunately the picture is terrible quality - it was recorded using an RF aerial rather than via cable TV. My sister had in fact not set up her TV before as she had recently moved. But when she turned the TV on and saw an old interview with Mark and Harrison, she knew she had to record her for me - even if it's only bearably watchable.
So, has anybody else out there got this documentary!?