Thanks Data, for helping me diagnose my sick PC. 

The new PSU has sorted it, sort of. Still a bit unstable as it crashed whilst I was using it and wouldn't boot saying memory problem. A bit of a wiggle later and it seems fine. Will give it another clean out later.

Snowy, have you tried switching off and on again. 

Excellent, well apart from the memory, yeah a good clean might sort that 

What PSU did you get in the end?   

Diesel, the good old turning it on and off again didn't do it this time    but I assure you it was tried many times 

I got the OCZ 750W. They actually had the 550W and 650W in stock too.

A modular one would've been neater but hey, it's working. Mobo is still a little twitchy. Will clean it again, wiggle it and switch it off and on again. (I call it a full manual reset)

That is a monster of a PSU there Snowy, should be big enough for your needs, just 

If the memory keeps dicking you around you could try it on one stick in single channel mode, then you have the option to test each stick and slot independently 

I call turning it off and on again a "hard boot" as apposed to a "soft boot" which would be a restart without turning off the power.