Ive had the same thing with a audi usen the reg off my sapphire.. i reported it to police and DVLA both waste of time but at least it was on file if you like... But i contacted the company who sent fine and told them i got few letters saying it will goto court but nothing since.. in my case tho were talkn about a black audi usen a reg of blue ford sierra thats been off road for 5 year with no mot.. So there not going to get far... How ever if it was a blue sierra useb reg i feel it will be a right ball ache in life tbh