I have found how to get it to install, I removed the other (SATA SSD) disk drive. It now boots successfully, phew!
I've no idea why that second drive breaks things. I installed it when I was still running xubuntu 24.04 and that OS could see the drive OK. I actually copied the whole of my old (xubuntu) installation across onto that drive. I will try putting it back later to see if it breaks the Debian 12 installtion but for the moment I'm just relieved I've got it working at last! -- Chris Green
