Oldbiker1950 has got a Yamaha TR1 which is in need of major restoration and he also has a Rickman Metisse frame which was obtained from the factory in the late 80's when it closed down.
“The frame is in an unfinished condition which means it was never allocated a frame number. I know that I cannot build a "special" and get an age-related reg, only a Q plate which I don't want. But could I use the TR1 as a donor and retain its reg no. for the new bike?”
The short answer would be “no”, the DVLA regard the frame as being the bike therefore a new frame usually warrants a new number. Without a serial number it would be impossible to prove the age of the frame.
You would also have to prove the identity of the frame, ie provide original factory receipts so that it isn't a stolen frame with the numbers ground off.
Prior to registration you would have to be submitted to a DVLA identity check at an approved centre, followed by a further SVA check, then an MOT.
The Q plate is the easy option, usually applied to vehicles of unknown age or made of parts of other vehicles of mixed age.