Write back to the DVLA, told them when you sold the car, who to, and his address.
I had this problem a couple of years ago when I traded in one of my cars at a dealer.
The dealer sold it on, and the 'new owner' not only didn't tax it, but parked it everywhere he could. on double yellows, in disabled bays etc etc.
I had a load of correspondence with the DVLA and two councils, but it was only after I suggested that they remove the car and crush it the next time it was ticketed that they started to take me seriously.
Cost about £4 in stamps, but I got the fine dropped in the end.