Ok, quick update on this.
Using the latest non-beta BIOS (F18 - beta ones don't like my keyboard and mouse), I used the above settings (post #2), but it's still doing it I'm afraid, most especially on Tomb Raider (did it 4 times last night). Do I wind the DRAM V down to 1.35? It was on Auto before. (Also I can't get 1.55 exactly, it's either 1.54 or 1.56). Ran Memtest86 three times before messing about with voltages etc and that never reported any problems.
I'm also wondering if my aging GTX275 is to blame, could the Z77 mobo, RAM or 3570k be reacting to the older piece of hardware? It has a factory overclock on it too. What would the reference speed be for a GTX275?
It seems to do it if there's a lot of pressure on the graphics. It's BSoD'ed 4-5 times for a very detailed Train Simulator 2013 route (lots of assets etc to draw), once for a very heavy Minecraft map, and now 4 times for Tomb Raider (all in the space of 5-6 hours yesterday).