Have you checked out the price of a second hand frame with log book! Is the engine number recorded on a cat b as well?

The V5 stays with the frame number. You can swap engines on the document ;)
A frame with a V5 usually fetches upwards of £600 or so............
eBay usually shows a few every now and then. Personally, whilst I see why some people break perfectly good bikes for spares, watch out for ones being broken due to a tpft accident where the owner can't afford to repair. I'd only buy a frame from a "known" source.
It should be possible to HPI a frame against the chassis number and V5 document though.

It used to be the case that a second hand frame with log book cost four times what a new frame from the manufacturer would cost. Why? Well someone goes out and nicks a bike of the same type then swaps all the bits over. They then have a "legit" bike with frame number and matching log book.

I didnt think salvage dealers were allowed to sell complete cat b bikes............thats just it with cat b, its to be broken, can be sold as spares, but not the frame!

What damage has it got? If it is a genuine cat b then i would suggest there is a lot of damage to expensive things, wheels forks etc let alone the frame.

They often turn up as trackday bikes andy. Skidlids has built a few. Loads on ebay.