My understanding is that they're not enforcable unless they can prove the owner of the vehicle was responsible for the ticket. They are enforcable but different to a ticket issued by a "traffic warden". It's all about the contract, which was agreed and entered into when the car was parked but they don't know who the driver was and can't prove who the contract was between.
Plenty reading online -
http://www.ticketfighter.co.uk/private.htm
http://www.moneysavingexpert.com/reclaim/private-parking-tickets
That's a pint you owe me :wink: