You can't be prosecuted if you are not the person named as the owner of the vehicle and you have never owned the vehicle concerned. You can't be prosecuted for not giving information you don't have. All you have to do, as people have suggested further up this thread, is return the letter to the address it was sent from, marked clearly "not known at this address". If you want to go into more detail, then write a letter being specific about the fact that you have no knowledge of the person concerned or the vehicle concerned. That's all. No need to panic. No need to worry.