If a company accept responsibility to store a car (or anything esle) they have a duty of care to protect it, In law this is a Tort - negligence, Ie - they did not do what they should have done or they did something they should not have done - this will be a cut and dried case for a small claims court against the OPC.
They cannot avoid liability and it does not matter whether or not they were receiving payment for their services, nor does it matter if an individual employee agreed to store without permission of the manager - the company are liable to the acts or ommissions of its employees (vicarious liablity)
A quick chat to the OPC of the relevant points of law will see them rectifying the problem, if not i would be more than willing to advise on the legal points in order to present a case to the small claims court, costs are very low to launch a claim (£100 ish) and any reasonable costs can be reclaimed when you win the case, The small claims court does not allow a company to engage the services of a solicitor to represent them at a small claims court to prevent the David and Goliath sundrome and evade justice.
Try talking to the OPC first and explain your position, mention the points of law and that you are prepared to use the courts, The small claims court requires that every effort is made to resolve the sittuation before a claim is launched otherwise the degree of compensation is reduced.
Glyn