Black Screen in Windows after Restart

Subject: Black Screen Of Death on Reboot and Unable to Update BIOS

I am having two issues with a B860M-H2, Ultra 225F, 32GB DDR5 ram. The first is that Windows 11 has been installed successfully but on each restart it will Black Screen of Death (windows will run but the screen goes blank due to no signal). The second is that I am unable to update the BIOS to any version other than the one installed (1.12)

Can I add that I am no newbie and I understand about memory training, discharging capacitors, checking and changing cables etc. I have had similar problems to the above over the years but they have all been resolved and this one has not yet!

Windows BlkSOD
Windows 11 installed successfully. When I restart (or shutdown and power up sometime later) the screen will go off a few seconds later. I know Windows is running because the HDD (970 Evo Plus) red light flashes on the case. All the fans are on, and the keyboard caps lock light is toggleable. Although I try Alt-F4-Enter to shutdown but it does not do that.

The only way I can 'get a screen back' is to shutdown (hold power button for 5 seconds), turn off PSU, and then clear the CMOS via the jumpers. I discharge the caps by holding down the power button and then I power up ... I can now get back into the BIOS screen but if I then restart (with our without changing any settings) something different happens...

No POST
This is now different. What happens is that the PC restarts, all the fans come on for about 3 seconds and then clunk - off. Everything turns of for 1 second and then the CPU fan and AIO pump turn on but the case fan stays off. There is no power to the USB devices (no caps lock light).

Remove Disk
To ensure Windows is not doing anything here I remove the 970 and try again. Exactly the same happens. On power on there is 'full fans' for about 3 seconds, and then the CPU and AIO pump go half speed, the Case fan turns off. Nothing - no screen, no POST, no splash screen nothing.

The only way out of this is to reset the CMOS via the jumper. I can do that ... get into the BIOS, I can change things or leave alone (usually I restore factory BIOS settings) but after a reboot or two I get this no POST.

Updating BIOS
Next I try a BIOS update. The onboard version is 1.12 so I try 1.14 and the 2.03 Beta. But both fail.

Flashback fails because it does the solid green light thing. I power off, insert USB stick in the Flash socket, press the button, flashes a few times then goes solid. I try all USB sockets to no avail.

Insta Flash part works (note I have done this via F2 and the F6 shortcut method). If I try to flash 1.14 or 2.03 it gets to 10% and then says 'After pressing enter the system will restart...' which is a fail because it does not say Success.

The only BIOS I can successfully update is the one I currently have. 1.12 works but not the others.

I am really stuck with this. I don't know if the combination of components do not work (the CPU, Memory, NVME are all on the QVL) or if it's the GFX (don't have a spare to hand) the PSU (BeQuiet Pure Power 11 600W) or anything else with the case (have removed all devices and jumpers apart from the power switch).