What was the reason giving for imposing the penalty charge? Was it, for example, for parking in a disabled bay without displaying his badge (even though he did)? Was it, perhaps, for not displaying his ticket (even though he did) or was it for over-staying the time allowed?
Also, did your OH check that the unrestricted parking for £2 applied on the day that he parked? (Car parks run by Ipswich Borough Council, for example, offer unlimited parking for £2 after 2pm. However that offer only applies on weekdays. Someone parking on an IBC car park on a Saturday might not realise that the offer doesn't apply then. There might be a similar limitation on the car park your OH used on market days or certain other days).