Mike
The fact that there is not trace of the registration number on the DVLA database online check does not mean the bike does not still exist.
Starting in the mid 1970's when you 'taxed' a vehicle with the local licensing authority or post office, using the RF60 / VE50 old buff logbook the logbook would be cancelled, the vehicle details entered on the then new computer system and a V5 registration document issued by DVLA.
If a vehicle was not 'taxed' during this period its details would not be entered onto the computer. In the 1980's DVLA dictated that anyone with a vehicle that had not been road taxed and issued with a new 'computerised' V5 registration document should apply to DVLA and exchange their old logbook for a V5 document. The vehicle details would then be entered onto the computer system. They allowed such 'updates' to be carried out until 1983.
So if the owner did not request a new style V5 document by 1983 the vehicle details would not be entered onto the computer system and the authority to use the registration number on the relevant vehicle would be lost.
There was also a period in the 1990's where the computerised details of vehicles that were on the computer system, but had not been 'taxed' for 3 or more years, were removed from the main system (they were running out of computer storage space) and put on a backup system which is based on microfiche records.
DVLA will reinstate a vehicles original registration details even if they had not been informed that it still existed prior to the 1983 cut off date. In order to do this they must be provided with the original buff logbook and the vehicle must be inspected by a recognised body (usually an owners club) to make sure the logbook (frame / chassis number) does relate to that vehicle and that the vehicle is substantially composed of period parts. They will also accept other official documents if they bear the registration number AND frame / chassis number but are very strict on what they will accept.
So the bike may be sat somewhere waiting to be rediscovered or it may have been exported without having been entered on the DVLA computer system.
Cheers
Alan