This has been discussed at length here on AB , these are not 'fines' or 'penalties' they are an invoice for parking nothing more. The issuer would need a county court judgement to enforce the payment and even then it is likedly the court would only award a nominal amount. There was a case recently, Oldham I think it was, the parking company took the case to the county court and was awarded £1 and had to pay all the costs, it had already cost them £1500 to take the case to court then there were court fees on top , there have been no more cases since.
Tell them 'Sue us and be dammed'