IT'S ALIVE! :-)))
After swapping the CPU, I got 8 beeps/flashes, and it fully reset. (Switched off and back on after a couple of seconds)
Came back with a lot less fan speed.
At first nothing more happened, although I could hear it reboot (the slight chirp from the speaker when it warm-reboot)
Decided to switch it back off and swap out the temporary video card for my regular card (GTX1060) as I wasn't sure that old card did actually work.
Switched back on to full fan speed. Again the 8 beeps 'n flashes. Again the full reset to a much quieter machine. And now the beep from my monitor indicating a wake from standby...
HP logo; flashing rom!! ....... 100%
Removed the stick, system rebooted, and came back to a repeating 10 flashes/beeps.
Switched her off, put back the emergency jumper to the normal position, and put back the password jumper.
Power back on; BIOS screen version 3.94!
Thank you all so much!! I'd almost started shopping for a replacement mainboard :P
Now if you'll excuse me; I've got a CPU and a metric sh*tload of DIMMs to reinstall ;-)
(Hope that other CPU still works though...)