The truck is about eleven years old and although in a very clean and straight condition may have been misused from time to time.
I'm inclined to agree with tom w with his opinion that the battery voltage may have been low or the truck was used to push some equipment.
At the same time I'm puzzled that all of the heating is in the inside of the controller and nowhere else and was trying to reassure myself by trying to find out if there was an internal problem with these controllers.
Anyway at this stage the controller has to be replaced and the owner has deferred repairing it because of the cost.
Many thanks to everyone for your opinions and help.