I would like to add to tatanab's comments about Tom Avon frames. The framebuilder's name was Steve Whitchurch. He did, I think specialise in tandem frames. We had ours built in the early 1980's. In the Bicycle Times issue 43 there is an article about him titled " Tandems by Tom". His workshop was at 7 Chessel Street, Bedminster, Bristol. I hope that this is of interest to you.
http://forum.ctc.org.uk/viewtopic.php?f=5&t=30434
Looking forward to the build!