No V5 yet but the bike does exist. Found I could do an enquiry on DVLA site which told me make, model, date last RFL etc, date last V5 issued. And it matches what I have been told so far.
Things don't look too bad then! I'd still check the bike over very carefully as it doesn't stop a stolen frame/engine being used to rebuild a legit bike - but that's true of any bike up for sale.