So this let me get on with testing the tool power motor. Long shot is that it works and can be driven cw and ccw at a controlled speed - I think it's 2 HP so Bridgeport sort of power.
Along the way a couple of irritations happened:
Firstly that temperature relay failed again for no good reason - I temporarily linked it out of circuit and got on with it, but I've just again dismantled it, cleaned the relay contacts, re-soldered the pcb joints and put it back with no definite fault found apart from it can only be the relay contacts or wires going to them, as the relay was energised when the fault was present.
Secondly the monitor screen went blank, then came back - loose mains connection found in its sealed box (only 14 screws and a gasket

)
As I was testing the drive, I also played in MDI mode starting and stopping the main spindle - hey what's that odd clicking noise

It turns out that while a program is running the amber beacon that I repaired yonks ago lights up, and when it ends it flashes - the clicking was its relay

Have an amber beacon: