I would appeal it to the adjudicator. They tend to take a common sense view of these things and an overheating engine can happen to anyone, believe me. Even if you lose at the adjudication you won't have to pay more than the original charge, I believe, so it's probably worth a punt.

My friend, a student at Canterbury Christ Church university, parked his car several times in one of the university car parks without incident, until one day he got a ticket - and then another. In line with advice I have seen on this site, I advised him to ignore them. In time letters arrived, increasing the penalty from £70 to £140 for each ticket. Again, I advised him to do nothing in response to letters from the security company. Eventually, a letter from the university itself arrived, which said that if the amounts remained unpaid then his diploma would be withheld when he graduated. Can they do this to him? They have no proof that he was driving on those days - it could easily have been me. I would be grateful for any advise on the legality of their proposed action.