Tried an additional BIOS change.
Went to "AC BACK" setting and changed from "ALWAYS OFF" to "MEMORY".
Continued restart into Windows from BIOS.
Shut down PC from Windows.
Restarted PC from Power Switch, same post error, flipped PSU...
Okay, I think I am getting the picture now. The GTX cards had the 6 pin PCIE power connector which do not have the Sense1 pin.
So it looks like a 6 pin SATA to 8 Pin PCIE power adaptor should be tried?
I noticed that on for the cables that came...
The Problem Is Solved (Sort Of)
Replaced PSU today.
Started PC from cold start. No Problems. No memory error beep codes, no GPU error codes.
Shut down PC from Windows. Re-started. Problem returned, requiring PSU to be switched off then back...