The signs on site are Minster Baywatch. Have you approached the retailer where you spent your money and asked them to intervene and get their landlord, who is the likely contractor for the parking operator, to get it cancelled. This is know as Plan A.
If they won't or can't help, then seek out the managing agent for the retail park and contact them and get them to cancel the charge with their agent. Complain to the retailer and have them escalate that up to the managing agent if necessary. Threaten to return the goods and go elsewhere to make the same purchase, explaining that you don't expect to be invoiced by a third party for the privilege of being a customer.