OK, here's where I get out of my depth! I don't really understand all this BIOS stuff. However, I can give you some answers, as someone else did suggest looking in the BIOS.
When I go into the BIOS, I look under Settings -> General -> Boot Sequence, there is a section titled Boot List Option, which has two radio buttons, Legacy and UEFI. The Legacy one is checked, so I guess it's not in UEFI mode.
I did try clicking UEFI and reinstalling at one point during my long and sorry saga, but it was a disaster. The main drive wasn't recognised, and nothing worked. I ended up reverting the BIOS settings back to their defaults and reinstalling.
Don't know if that helps or makes things even less clear! Thanks for the reply.