Per (PeteCresswell):
I was not ruling out RCI on my part - my assumption having been that if
the connectors plug into the mobo, it must be the right kind of PS.

Bingo! Either RCI or a bad second PSU.

Put in another PSU and it came right up.

After, if course, I had hosed the BIOS settings by replacing the CMOS
battery - just on GPs... -)
--
Pete Cresswell