Just an update..
Replaced the DHW thermistor and it still over heated on hot water.
I then ran the F3 cleaner through it buy closing off the flow valve with the hot water tap open. Ran it for about 20 mins with the cleaner in it, left it for half hour and ran the tap again for another 15mins. It still continued to over heat as I ran the cleaner through it. Gave me the impression it hadn't worked. Flushed the cleaner out, which was quite black, filled it up to 1 bar and ran it. At first it still didn't run right, but eventually it settled down and now seems to be fine. Customer has used it for a week and said it's perfect. It makes me think the main heat exchanger was blocked.