Most of the advice given is correct though I could not advocate taking some of the action suggested.
If the parking notice was issued by a private company and not a local authority the theory is that the driver of the vehicle, provided there are signs as to the maximum parking period, has entered into a contract and the driver has breached the contract, however the remedy for breach of contract is first court and, if successful, damages. It is very unlikely the parking company will take you to court.
If you received the notice as the registered keeper of the vehicle the parking company may ask you who was the driver of the vehicle, you are under no obligation to provide this information, if you refuse they may talk about a Norwich Pharmacal order, to force you to provide details of the driver (the origin of this case in 1974 has nothing to do with parking ), I have dealt with a number of these cases and have never known the parking company obtain this order as they must first go to court which they are unlikely to do. Remind the company of section 40 of The Administration of justice act 1970, which may make it an offence, under certain circumstances, to coerce payment by using means which may cause harm or distress.