Happened to me at B&Q too, I parked where I should not have, aparently (no, not in a child space or disabled space, but in a quiet dead corner, my motorcycle, as they did not have designated motorcycle parking bays). I refused to pay and they set the bailiffs on me and I still refused to pay and the bailiff accepted my legal template letter and have not paid to this day. There are on line customer / consumer rights websites run by people who know their facts, which are very very helpful!