Thanks Casey. A CMOS reset seems to have helped, although I have also removed the graphics card for the time being. (I was actually borrowing the GPU from my existing, working machine. I've a new one on order so will install that when it arrives next week.)
And I think what was certainly adding to my grief was an NVME drive with a completely empty EFI partition - it seemed to not like that at all. I don't know if that's normal/expected but it was basically my fault for rushing things.
Can I just add that the instructions here are *outstanding* and the help is very much appreciated. Thank you