I had a G5 CB250, totally complete, 2k miles, original everything but like Mick's it had been off the road since the late 70's and was not on the DVLA register but I had the original one-sheet V5 doc. DVLA insisted on a dating certificate and told me the bike would need to inspected. Contacted Ray Davis who is the VJMC dating man, sent him tons of hi-res pictures and he told me that one of the frame digits was corroded to the point where it was unclear if that digit was the same as the one on the old V5. Sold the bike as a project and offered to get everything sorted for the guy but he preferred to do it himself so I let him get on with it.
I suspect the best thing Roy is build the bike up and then apply to DVLA