By your own admission you did not conform to the contractual terms of the parking controls. You should have first made an arrangement with PPS to park your car there. See: www.honestjohn.co.uk/faq/parking-fines.

However, this does raise the question as to how anyone living in a flat with controlled parking can ever get any work done on the flat by tradesmen who have to park their vehicles outside. It seems to have reached the point where normal life as we know it is impossible simply for the sake of the profit of members of the British Parking Association. This is why any attempt by the BPA to extend its powers against keepers of cars needs to be countered by careful regulation of its penalty procedures, an ombudsman appointed to deal with disputes, and the Protection from Harassment Act 1997 heavily enforced against BPA members. So far, the proposed bill only limits fines to £150 with a 50 per cent discount for prompt payment. That kind of penalty will impose great hardship on many pensioners.