dont panic, all might not be lost,
the registration number always remains the property of the DVLA ( little known fact ) BUT you should still be known as the grantee - unless someone has falsified a signature on cherished transfer paperwork.
Its going to be a right ballache to sort out, espeically now there is no loacal DVLA offices, but I would write to DVLA, explaining it all, and stating you would like the plate to be put on retention.
Good luck, your going to need it, cos the latest owner could kick up a fuss and will have to pay his insurance admin fee if nowt else IF you manage to get the plate off his car