You're welcome, happy to help. It's a good idea to contact the parking company again and request a copy of the decision notice they claim to have sent you. This will help you to determine whether they followed the correct procedures for notifying you of their decision.
If you're not satisfied with their response or they fail to resolve the charge, you may want to consider making a formal complaint to the BPA. As a member of the trade association, the parking company is bound by the BPA Code of Practice, which requires them to follow certain rules and standards. If you believe the parking company has breached the Code of Practice, you can raise a complaint with the BPA and they will investigate the matter.
You may also want to seek legal advice if the matter remains unresolved, as there may be options available to you for challenging the charge and the debt recovery company's demand for payment.