Get in touch with yer local friendly plod office, a bit of advice though dont go on a busy weekend evening, they'll be too busy t give ya a decent answer! A mate o mine had this problem with a rallycar that he bought, he ended up having to register the car wi a Q reg cos he couldn't proove the origins of the shell that it was re shelled with. You are only entitled to use the original reg number on a brand new un-registered shell, whereas a second hand pre registered shell would mean you having to proove where the rest of the major components of the car came from in order to give the car an identity. The DVLA website tells ya all about it, I looked in to it when I was doing the same thing mysel, I still not got the car on the road so havent done it myself yet. Good luck wi the reshell m8