Unfortunately mate these things happen.

When I sold my runabout Volvo last month I did the transfer prior to sale - as the new tax disc/mot etc hadnt come back the car went to the new owner with my private plate on but obviously everything was in progress to transfer it onto my Omega. The lad met up with me a fortnight later so we could swap the plates back to the originals, but if he had ignored me etc then I would have been able to take action as that plate showing on the Volvo would have been registered to a completely different car !!

I did give him a receipt stating that it was sold minus the plate however (also to cover him due to the fact I had no log book whatsoever as the DVLA had it, it worked this time but some people might be funny about selling a car without a log book )