Thanks Eggy1968
Worked for me and I understand it much better now.
BIOS compatibility mode was essential to get my non-uefi system drive recognised. Then conversion of the system drive to uefi using Microsoft tool MBR2GPT.exe (via run with admin permissions in the still running Win10 environment) went like a dream. Turning off Compatibility Mode again was the trick to get the display port graphics going in during the boot sequence.
Rob