Dealer (Arthur Daley type) is now saying he's lost the contact details of the guy.
Basically I've been stitched and have nothing in writing to say he agreed to return my plate. Its not worth a lot but thats not the point.
What I should have done is to transfer prior to sale. All I thought I needed was the V5 which I *did* keep to do the transfer. What you also need (which I didn't know) is tax and MOT which went with the car. So its basically bye bye unless I can find the new owner.
New owner is a policeman from the midlands. I just hope he's got internet access as he'll soon find the register.