PSU
I did a swap out with wifey's PSU.
Booted up just fine.
I'm finding, nowadays, that as long as the machine is running - it stays running. Only had one more full freeze since I last wrote.
If the machine turns off, however, it's an absolute pig to get going again.
Bit the bullet when I'd made some space on my credit card - went off to Newegg and bought a 700w PSU.
Gorgeous piece of kit. One of the "modular" things with lots of output sockets on the back and you just plug in short looms to facilitate what you need and nothing more.
"ModXStream Pro" with "OCZ Technology" (no idea what any of that means).
Installed in minutes, very neat, stunningly quiet. Great piece of kit. US$95 (£59).
Turned the machine on......nothing!.
Guess it wasn't the PSU then.