According to the DVLA you are still the registered keeper. You will need to inform them of when and who you sold the vehicle to.
The registered owner is responsible for any Penalty Charge Notices (PCN) issued to the vehicle.
Email enquiries to Parking - CSC
By post: CSC (Parking Section), RBWM, Town Hall, St Ives Road, Maidenhead, Berks SL6 1RF