Hopkirk. On 1st October 2012 there was a change in the law re pricate car park operators. It now allows them to pursue the registered keeper rather than the driver of the vehicle. Also clamping on private car parks became illegal on the same date.
Also private operators cannot impose fines or penalties.
The topic was covered recently by BBC Watchdog. It may even still be available on iPlayer. In the meantime, they have this page, with some case histories and responses from a selection of named parking companies.