The sharp-witted amongst us will have noted in my previous blog that I had found the cause of all the problems with the RigidBot, but I didn't recognise it even while I was writing about it.
The open-circuit on pin 1 of the extruder flat cable is a sufficient explanation of all the problems the printer showed. I simply didn't follow an orderly chain of testing the components and cables to isolate the cable.
Anyway I've ordered some replacement cable and connectors and I will now have to wait the three or four weeks it takes will they arrive.
My hypothesis is that I will be able to return the original main board to the printer, replace the faulty cable and everything will work correctly. And if it doesn't, I now have so many replacement parts that I can replace nearly all of the fragile parts of the printer to keep it running.