Thanks.
Yes, I have been looking at that. I had previously taken it off and remade the lug on the end. It all looked good with a low resistance but I cleaned it up anyway.
I did find that two of the earth braids a n the screened cables had failed. The job for today is to remake these.
It's definitely a wire fault. With the ECU it situ the mini wouldn't crank. However if I held it in the crank position and moved the ECU then it started!