Gosh, good luck finding the recording :)
Some
possibly useful things... although in this case I've
drawn a blank on them so their use is probably limited to saying 'don't
look there then' ;)
1. The
BBC's excellent
Genome Project
website lets you wander around issues of the Radio Times published
between 1923 and 2009. This lets you find out the time / date / channel
that a particular programme was broadcast on, and repeats. I suppose
it's possible that someone setting up their VHS to record a later or
earlier programme might have captured a bit too, so it may be useful to
know what preceded and followed the broadcast.
2. Given the subject matter might university linguistics departments have a copy?
4. More
obliquely - I'm fairly sure I've seen snippets of David's Ri lecture
embedded within other programmes. Possibly rights-clearance information
would point to something although obviously it would be tiny fragments
rather than what you're actually after.
5. Even more obliquely -
Home Movie Day - an event around the world where people attend community events and show old home movies. In London the
Cinema Museum hosts a day of films, tea and cake. It struck me that the sort of people interested in this might have been recorders of other things and possibly have dusty attics full of stuff.