This was the latest reply I was advised to send as the appeal:
Dear Sirs,
I am appealing this parking charge notice (PCN #: _____) as the registered keeper of VRM: ____ ___. I am appealing on the grounds that the driver did not breach any of the terms of parking as advertised by the signage on the site.
Your parking charge notice alleges that the driver incurred a charge by "either not purchasing the appropriate parking time or by remaining at the car park longer than permitted". The signage at the site (a copy of which is attached) states that tariffs only apply between 5pm-8am Monday-Friday, and all day Saturday and Sunday. As the car was not parked during these times, no tariff was due. Additionally, the signage does not specify a maximum stay outside of the periods when parking tariffs apply.
Accordingly, the alleged breach did not occur, and the parking charge is not due. You should therefore cancel this charge, or issue a POPLA code.