I put up a request for any information on this coach, and the thread went dead as soon as it went alive. A kindly soul texted me with a few photos, and I thought it would be handy to post it here, if I got hit by a bus etc.
"To stock 1982, Grill/Bar/Dining Car
546 ex 5970 (TSO) and RSS 1800 - Mk 2f TSO: converted 1974 to prototype self-service buffet for APT-P."
This is most fantastic bit. They took a stock coach and fecked about with the interior to see how it would work as an APT coach. It didn't work, so they reverted it to RSS.
RSS you ask? Restaurant Self Service - a version of motorway services, where you perused and purchased and turned up at the till. Sounds mighty miserable.
"To Northern Ireland Railways 1983, regauged to 5' 3". When withdrawn, repatriated regauged as No. 1800. 2003 Overhauled at Heysham to become part of Wessex Trains Excalibur set as 1800. 2009 now based at Carnforth, although still operating as part of same set."