I have wondered in the past when doing this sort of search if a further possibility is that the number has been transferred to another vehicle.
That the DVLA site requires make and number will mean that it will show as 'not known' if that number now sits on, for instance, a Mercedes Benz.
Once a number has been transferred, it then becomes almost impossible to trace the original history, even if the bike is now still running on an age-related plate.
If you know the frame number, you could try advertising in the specialist press and if you have lots of time to spare, you could search the DVLA site for TOF 109 under every make on their list to see if it is still current.
The days are gone when it was possible to get a friendy Policeman to run a search. It is now literally 'more than their job's worth' Maybe a wheel-clamping firm would do it though ?
The moral here is to never sell a Norton - you'll always regret it !