I think I'm going to have to ring Asus tech support tomorrow on my day off, I can't see anything wrong on mine, but must be something. I was hoping it was just something small and after a few clicks it'd be sorted.

Easiest way to decipher what the issue is, is by looking at the right side of the easyBIOS when it its in EZ mode.

It should have all boot priorities listed with the windows boot manager being listed first alongside the name of the drive its on. If it is listed but not first, then simply drag it up to first in the list. if you press F7, you can also navigate to the furthest right option, which i believe is save / exit etc and there should be a boot override option in there which will show the windows boot profile if the unit can indeed, see it on the drive.
If you can't see this then the chances are the drive has gone kaput.

If you can see it but it instead only lists the drive and not the windows boot manager, then the boot profile has been wiped probably from an erroneous MS update.

Mine has gone back to Asus, luckily its young enough to be under warranty still. Not happy to be without a laptop for a few weeks though!
The guy at Asus tech support concluded my hard drive had gone wrong.