One of my clients has a problem - her car has been blocked in by a member of the public using the centre's car park without permission, and as a result she is unable to go home.

What actions can she take to get the car legally moved out of the way?

Will AA/RAC help? Will the police bring in a tow truck if requested?
Can a private towing company be instructed to move the car out of the way, and if they want to be paid take it up with the car owner?