Just an update on this. Sorry it's taken so long but using PC every day so not had a chance to try and sort it until today.
Reminder of the problem: Couldn't disable fast boot so flashed main bios, which left me with a PC that would only boot on the backup bios. Trying to boot on main bios would just give me a flashing cursor on a black screen.
Today, after preparing a bootable usb stick containing the Efiflash utility and Z87XUD£H.F9 Bios file, I started the PC on backup bios then pushed the bios selection switch over to main bios. Booted to the usb and flashed the main Bios.I now have a working main Bios again. But still cant get the PC to show the Gigabyte post splash screen, allowing me to press 'del' to enter the bios. Again tried using the clear Bios button and removed the battery for several minutes. Gigabyte app centre is still telling me that fast boot is disabled, but still boots fast without the option to enter the Bios.
I'm now wondering if fast boot is actually disabled and its the splash screen that's been switched off in the Bios. I may be wrong but I seem to remember that disable splash screen was a separate option in the bios. Can't be sure as I'ts been years since I played around with the Bios options.
Anyway, whatever is selected in the bios, I would have thought that re-flashing the bios would have set it back to default settings?