As I understand it, unless the ticket is issued by a council officer of some sort, a traffic warden or the police, then the contract is between the car park operator and the driver (not the registered keeper) of the vehicle.
So, when the letter arrives at the registered keeper's address, either don't respond at all, or reply saying that you are unaware of who the driver was at the time. This excuse doesn't work with the police (speeding fines etc), but should work in this case.