BR,
Thanks for the detailed reply. It is appreciated and I'd meant to respond before now.
I have arranged to borrow a power outlet tester, so in the very near future I will be able to verify my 240V 50Hz supply is good.
No doubt I have lots to learn about grounding, and as you have suggested I have started again.
You say I must have crossed "the DC common and grnds somewhere" since I can measure voltages between stepper bodies and supply ground. But I expect you will always be able to measure a small parasitic DC voltage between stepper body and ground when the motors are energised and not grounded. When energised, steppers are always drawing current so in the presence of permanent magnets a voltage will be produced.
I am going to ground mys stepper motors and also isolate them from the machine (aluminium extrusion) body.
My controller is designed and made in Australia. It was a deliberate purchase to support a local business. God knows we don't make much in our country any more - only dig holes in the dirt. The IC3A controller is plug and play for many, many customers. But obviously my use is one of a kind. The provider has been very ill, and helpful to a limited extent, between hospital stays.
I will introduce components one at a time until a problem presents - thanks for the tip.