Well I put all 4 DIMMs back in, but swapped them round (As in, the plats were in channel 1 and the golds in the other channel, and I swapped them around so gold in channel 1 etc) and left memtest running and its fine.
So I guess it could have worked loose, or maybe something odd about having the fast ram in channel 1?
EDIT:
I spoke too soon, still crashing. Went into the BIOS and reset all the timings from auto to the slower ram speed and put the voltage to 2v (I'm guessing a possible culprit is the 2 lots of ram operate at different voltages)
Why don't you just run one type of ram with one type of voltage