I now have a copy courtesy of @crypticinquiry but I sincerely thank all of you great Mastodon peeps for sharing and helping in this. You've all made a sentimental fool very happy x
BBC Programme Index

@IanThistlethwaite Since you are already asking about "a long shot", I assume that you have tried the obvious sources?

The British Library radio archive has folk on 2 recordings from the week before and two weeks after that Christmas concert. I there a list of things you tried unsuccessfully?

(http://cadensa.bl.uk/uhtbin/cgisirsi/?ps=P5tOdN14Im/WORKS-FILE/198560043/18/X474/XTITLE/Jim+Lloyd+with+folk+on+2)

The British Library Sound & Moving Image catalogue

The sound and moving image catalogue containing almost five million recordings from all genres.

@mathias I suspect it might be a copyright issue, but I've looked a few places and it's not there so I think if somebody has a recording they made at the time and diditised, that'd be perfect but I also think they'd not pop up very easily in a web-search
@IanThistlethwaite Copyright should - if any - only affect the archive's ability to hand out copies. It should not prevent an archive from storing, digitizing and preserving a recording.