After poking around for a while he decided that the fault was on the circuit board, and they are VERY expensive to replace. I asked if he could remove the board so I could have a look; perhaps there was something obvious.
The picture shows a view around one of the connecting socket terminal pins. You can see that the pin has been making intermittent contact with the solder on the track and sparking (debris on the left of the pin). This also explains why a quick “thump” on the main unit would often bring it back to life for a while.
I took the board and using my inspection microscope found several other cracked solder joints.
It took me 10 minutes to inspect and re-do every suspect joint. There was also a resistor that had been overheating so I replaced that whilst I was at it.
All the bad joints were at the same end of the board and I suspect that this part of the board suffers from thermal shock due to its location under the actual heat exchanger. Whatever the reason, once the board was reinstalled everything started working normally and it saved me a £250 bill for a new board; a board I hasten to mention that has around £10 worth of components on it.
In the end when I asked how much for the work he only charged me for the call out as in his words, the repair was a “joint effort”.