I'm keeping my fingers crossed at the moment as it's been a while since I've had any problems.
I noticed that my RAM chips are slightly different in height, obviously a manufacturing thing. Anyway there is an option in BIOS which enables the RAM to be more compliant with each other. I can't remember what it was actually called; something to do with tolerance I think. Anyway, I've tried it and hope it works.
Thanks for your help and advice, much appreciated. I guess it won't be long before I'm here again, but you never know.