http://www.adviceguide.org.uk/scotland/consumer_s/consumer_cars_and_other_vehicles_e/consumer_driving_and_parking_e/consumer_parking_tickets_s/parking_tickets_on_private_land_s.htm
There's a better info site that I can't find now, it goes on to say about time scale, if the ticket came through the post too soon it can't be enforced, and not to correspond in writing without starting it with 'without predudice' because you could be admitting liability ect ect.
There are two places to appeal to but it seems easier to let them take you to court, which almost never happens.