Dave the dog is most likely correct in saying don't pay it - but:
*Was it private land or Council - real tickets that you have to pay will have the address of a court on them and will say 'Penalty Charge Notice'. Civil ones have the address of a private company and say something like 'Parking Charge Notice'.
If it is a private ticket, bin it. You do not have to ring them or quote legislation - just ignore it, along with any letters that follow, no matter how threatening they appear. Entering into discussion with them just shows they have a 'live one'.
Private tickets are based on contract law. To enforce them, they must prove there was a contract between you. Also note any contract is with the driver - NOT the registered keeper. Do not admit to being the driver under any circumstances.