The last crash was a total 'turn off'. The power button on the PC did not respond and, as I said, it only worked when I switched it on and off at the wall. It was this last one that made me think PSU.

The others all have different codes. One appears to make reference to the graphics card. When I reinstall new drivers you get a NVidia power warning, but I had read on the web that this was 'common' and found a registry fix. Now I'm not so sure it was so wise to ignore and wondering if that crash was related to the power warning.

I have updated the BIOS via Dell from VO3 to VO7. Still no joy.

Not sure if this will work, but here are the actual screen shots of the last three crashes: (Mods, if I an breaching any board rules, please delete):