Firstly, thank you everyone who tried to help, it was much appreciated, even if it maybe didn't sound like it (I was getting a bit frustrated).
As I reported, removing the second disk drive (a 2TB SATA SSD), allowed me to to the install with no problems at all. I had installed the drive when the system was running xubuntu 24.04 and it worked perfectly, I installed a backup of the xubuntu 24.04. Having got Debian 12 up and running I decided to try and put the 2TB SATA SSD drive back. It all works fine! So the presence of the SATA SSD was only preventing the Debian installer from installing grub/boot, now they're installed the system boots quite happily with the drive installed. Very strange! :-) -- Chris Green
