Further to my woes. It is now conclusive that I have a dead galvo driver.

I first swapped the power leads to both drivers to see if it was a power supply issue. No problem, power to both drivers.
I then swapped the X and Y at the motherboard - no difference other than the Y galvo now moved as expected.
Then swapped the final lead with the same result.

The only conclusion I can reach is that I have a dead galo driver.

The problem with the software hanging was my fault. I didn't realise the configuration window hides behind the main screen, when power to the printer is switched off, and wouldn't allow anything to happen before it was closed. So, just a dead driver and a faulty main switch to report to Sintratec.