I appreciate the reply, Ian.
I have been in communication with Keith this morning, and he has been immense help. (So nice to deal with a company that you can rely on to give you superb support on your product.)
I am satisfied that the IPL is fine, although my research does show that it can be an issue. In past work on the desk, I removed the connectors and soldered the IPL connectors directly on to the board so that this issue wouldn't arise. I did a continuity test of this cable anyway, and it checks out fine.
The desk is both Analogue and DMX, and the theatre uses both systems (hence this is a perfect desk for us).
I agree with what you say - It's a superbly constructed 'Tank' of a beast.
I am satisfied that both batteries are fine and operating normally.
I have tracked the problem down to the slave board (the one on the left as you look in from underneath). The tracking around the battery had very minimal corrosion, and I am happy that it is all in good order (but worth a mention).
I don't have a schematic, but logically tracking through the fault, I checked the 4051 analogue multiplexers that take the voltage from the sliders, and pass it in turn to the A-to-D circuitry. There is a problem here, as all the 4051's are giving 5V on their outputs, even though the input sliders are all down (i.e. zero).
This is as far as I have got.
Chris