I found this site today that not only has John Peel shows from '79 &'80 (never thought I'd find ones from that time period!!) but it has a clip of an actual Peel Session track by Nick Drake which I would have thought unthinkable I'd ever hear that because I recall reading that the BBC either lost that session or found it had been destroyed. Now I haven't heard the clip myself yet (I'm at work damnit) so I can't vouch for the quality, but my guess is it's probably just from a homemade tape someone made off the radio, so who knows.
Here's the site:
http://www.jonhorne.co.uk/jptapes/jptapes.html