OK - I dug into my BIOS and find there is an advanced setting to enable Windows 8.1/10 WHQL support. It is disabled. I have attached a scan from the manual below. is this the problem setting?
Is it safe to flip this setting with Windows 10 installed? Do I need to flip this setting then re-install Windows 10?
EDIT: Also found a "Boot mode select" function (see screenshot below). It is currently set to "LEGACY+UEFI".
It looks like I have messed up my Windows 10 install? Should I have set these options for Windows 10 from the start?