Quick Update on this as I've managed to spend some time on it over the weekend. Some notes
* took off the pump, dismantled and cleaned, re-assembled, no change, still working intermittently
* It already has been changed at some point with a reman one
* little tabs to the brushes and already been cut and were re-soldered
* The circuit board was pretty clean and the two main power transistors had been changed
* Some of the other chips have signs of heat - see pics
* Checked earth resistance and voltages on the supplies and it all seems good
* cleaned terminals
Interrogated with INPA:
* Had a couple of faults in INPA including an overheating one
* When it's not working, the control unit can be identified but it's error memory / live data can not be read
* when it is working, all the live data and memory works fine
Seems to be no rhyme or reason on when it does and doesn't work. Can toggle the ignition a dozen times and it won't work once, or it'll work a couple of times in a row.
Seems to be the control unit has just packed up. I'm going to contact the previous owner to see if he has a receipt for the replacement box.
Battery is reading about 12.5 when idle and 14.5 engine running. I've got it on a charge now to see if it will make any difference.
Not sure what else to do / try so any ideas are greatly appreciated.