Well another update!
Turns out the motherboard won't switch on with any decent PSU now. However a low wattage cheap one gets it fired up. So I guess something has failed on the board.
With a huge list of compatibility issues, flakey reboots and now this power issue, I've decided to ditch the board and buy an alternative brand. And guess what? No issues. Finally I can enjoy the 5820k processor without worrying if the machine will turn on, or whether I wont be able to boot past the BIOS screen anymore.
Sorry Gigabyte, I've not used your boards in years and have to say I'm not likely to again in a very long time. The X99-UD4 motherboard has been not only a catastrophic waste of money, it's been the worst board I've ever had the misfortune of buying
Unreliable, full of bugs and ultimately, poor quality. Shame