Railways (and ports/airports) are covered by an odd set of laws called Byelaws. They aren't made by Parliament but are generated by companies themselves, proposed to the relevant Secretary of State and then added to the body of law.
The last change to Rail Byelaws was in 2005. They cover parking and ask such parking charges issued by parking operators have to be "Regulation 14" byelaw tickets. They cannot be parking charges. Even POPLA has stopped accepting appeals on Byelaw tickets while they "clarify" if they can adjudicate on them. Some other parking companies continue to issue parking charges which are to all intents fraudulent as they have no basis in law.
Network Rail have confirmed the application of byelaws to all Network Rail sites in a Freedom of Information request. You can download a copy here and send it with any appeal. The letter is as follows