There is very good advice as mentioned by Dimond Geezer. Dont pay the "fine"; it is not a fine, it is an invoice from a private company. The advice on money saving expert is to write one letter in reply stating that you do not know who was driving the vehicle at the time it was parked on their client's property, and that as no criminal offence was committed you are aware that you have no legal obligation to find out, and that you decline to pay their invoice. Make it clear that you realise it is an invoice from a company (as opposed to a legally enforceable fine from the police/local authority.) When they write again, reply saying that you will treat any further correspondence as harassment and will forward it to the regulator together with a formal complaint about the company. I had the same thing from a parking company in the midlands a while ago and did just that, no problem since.