Tim, anyone doing framestuff in the UK (and any where else, probably) will have a Ceeway account. Whoever does the job should be able to get the part as well...
Now (seemingly after a test ride) achieved the use of a £15 Quad universal adapter bought from ebay. Slides in to the dropout with only a little reprofiling (of the adapter). Will give a full road test and post pictures next weekend if successful.
I knew a guy who was a Ritchey sponsored rider who had disc tabs brazed to his Team issue P21 for a trip to Moab. Problem was his skinny stays weren't strong enough - first big downhill, pulled brake snapped stays, broken collarbone and first flight home.