After three PSU swaps, two Video Card swaps, and getting no positive indicators with any configuration, I was ready to take to the box with an axe.
I looked at the replacement mobo battery that had been sitting on the computer for a month or more and had a moment. The time had been requiring adjustment every day for weeks and I wondered what else might have been going on, or not going on with a discharged battery, so I put the axe down and changed the battery.
All sorted and now running as per usual. I was too annoyed with myself for being tardy in changing the battery to be in any way chuffed with sorting the problem.
Lesson learnt and will be well remembered.
PS: All is not as per usual. I've obviously swapped some SATA leads as my Disk 'C' is now my old one with no storage available. Guess I'll have to play with it some more.