I sold the car prior to the ticket being issued, what should I do?

The reason why you have been contacted about the ticket is that you are still registered at DVLA as the keeper for the vehicle that was ticketed. 

You should contact DVLA to check this. If you have proof that you sold your vehicle you should first inform DVLA and then write to us including the evidence of transfer. Transfer of liability to the new keeper will only be possible if DVLA confirms to us that you were not the registered keeper of the vehicle at the time of the incident.