I don't know if he did keep any records of the bike and, unfortunately, he now lives in New Zealand so can't find out it's owner-history. I lost-touch with him years' ago and only found-out he was in NZ when I bumped-into him in Waitrose when he came home for his mothers' funeral, and that must be about 15 years ago.
Sorry not to find anymore details for you, but found another pic. It'd be great if it was the same bike, be nice to see it still going. Oh and, yes, the seat post was 'odd'. I think it was an SR Sakae. The idea was to fix a quick release underneath it so you could slide the seat back to put your weight over the back wheel on those steepe decents. I just used to stand-up on the pedals and hang-me-arse behind the saddle and scrape-me-piles on the knobblies.
My Super Maverick built at the Ilkeston SPD and in Team Raleigh Panasonic colour was his 'inspiration' to get the Avanti 'customised'. Here is a scanned image of it on the 1985 Muddy Fox Trailblazer ride on the North Downs Way at Dorking (beer stop).
Cheers