Hey Jack,
Working on the info you've been given I'd start by trying to track down any old school late 70's / early 80's roadies or trackies in van... maybe ask around at the velodrome any old school riders, or hit up ace at ace cycles as he's been around forever. See if anyone can confirm the name of the builder you've been given then I guess going off of that try and see if you have any way of tracking down his location, and with your fingers crossed maybe you'll find him and keep your hopes high that after 30 years he's still held onto decals for what may have been a very very small operation, or as you say just a one-off. Perhaps try contacting the previous owner and see if he has photo's of the bike in its present state from which you could attempt to recreate the original decals. Good luck.
Nick