sbiddle:
But the problem with taking this to the disputes tribunal is you run the risk of being asked why you were parked for 7 - 7 1/2 hours longer than you paid for. While the tribunal may rule the "fine" as illegal, the fact the OP overstayed his paid parking period by this time has to also be factored in.
I do believe the OP genuinly thought that he had paid for the 12 hour stay at $8 though. That could work in his favour.