It's the gigabyte ga-h270m-ds3h motherboard. PC switches on fine, motherboard lights up and all the fans are working. The gigabyte screen comes up but there is no 'press x for bios', which I've seen a screenshot for.

Anyway, I mash the delete button (which is what it says in the manual) from the moment I click the power button, but I can't get into the BIOS. It just goes straight to a black screen saying 'reboot and select proper boot device or insert boot media in selected boot device and press a key'. I know my keyboard works because when I click a key, that message repeats. I've also tried connecting the keyboard into every USB port available, including USB 2.0, but still nothing...

try f2 or f12

Your mouse and keyboard should be plugged into the USB ports circled in RED...





When trying to get into the BIOS, try tapping the delete key every 1/2 second or so.

As for the boot device warning message, it may be interrupting your attempt to get into the BIOS. Try unhooking all SSDs and/or hard drives (both data and power) then try to get into the BIOS.