RAM za GA-MA790XT-UD4P

Got 2 x 8GB installed and they worked!

Solved. The problem was the new RAM modules I had bought. For some reason they seemingly work fine, registering in windows with no detectable faults, but they wouldn't allow me into BIOS. I put the old ones back in and now it's fine. I still find this odd. The new modules were hyper fury x series 1866, 2x8GB modules. From what I read before buying them it seemed they automatically downgrade to 1666 and work. The hyper x series was listed as compatible and Gigabyte support knew which ones I was using and didn't raise it as a potential issue. Live and learn.