I managed to find the beta bios from 2013 on the Gigabyte support area which I think is the most recent version, extract it to a USB and then use 'Q-Flash' to install it on the motherboard while my old graphics card was fitted so that I could access it.
Allowed it to do its thing, booted to windows for it to auto install any new drivers and then restarted and seated new graphics card. Booted back up into bios with no issues, loaded optimised defaults, changed a SATA setting from IDE to ACHI (think that's what it was called - this is what the setting was with the old graphics card that I made a note of, anyway) and all is working perfectly.
Probably more luck than judgement but I seem to be sorted.