I'm clearly getting my numbers confused!
Before RMAing the board I was running TR3X6G1333C7 and it wouldn't boot from cold.
I'm now running TR3X6G1333C9 and it will boot from cold but it's wildly unstable. I get constant BSODs and programmes like Windows Explorer and Windows Desktop Manager are always crashing. Also seeing a lot of errors on my main HDD.
When I could get the C7 to boot, the system would be pretty much rock solid.
So I'll swap out the RAM again and see where I get to this time, I'm also going to update to the latest BIOS and see where that gets me...
The Champagne is back on ice (or in the garden as it's ruddy freezing here today!)