I reckon that you'd win that at a POPLA appeal for insufficient signage.This is the sign in the middle of the superchargers
This page provides information on tickets issued by Civil Enforcement Ltd and how to beat them either at appeal or in the county court
Keep badgering the hotel manager first though...they are the ones employing the parking firm. Get the CEOs name and batter him/her on Twitter and LinkedIn. Getting the hotel to cancel is the easier route, though make sure to keep proof as some parking firms have been known to issue proceedings even when the hotel have cancelled the invoice.