Meh...computer still not sorted.
First, my old power supply wasn't quite right, and though it would have worked, the relevant cable was too short.
So, I went and bought a new power supply on Saturday.
Then I set everything up, carefully and tediously injected the boot-critical drivers into my existing OS, and it all worked and booted up.
Then, it turned out that the new gen chipset for the motherboard has no Win7 support. So as far as the PC was concerned, I had no USB ports (so no mouse / keyboard) and no network ports.
So, resigned to finally installing Win10, I tried to boot up on my Win10 installation USB. But, the system didn't see any USB ports, so it wouldn't boot off it.
So I said screw it, and ordered a 500GB SSD to do a clean install.
And then it turned out that my Win10 drive was broken.
So I've had to make a new Win10 installation USB today at work. But when I get home, I will have no power. ::sigh::
Anyway, this went from replacing the MB and CPU to a seriously expensive build.
On the plus side, when it works, it will be the most powerful PC I've ever had.

And the most expensive.
--A