It is the 620...
I don't have any definitive answer yet, but all I know is there was a loud pop, and everything went off. I checked the mains plug and the fuse had blown, replaced it, nothing.
I guess whatever happaned blew the fuse, and it didn't work like its supposed to, you know the fuse blowing first to prevent damage...

There was a very feint smell of burning from the back of the PSU, but only very feint.
Now it did something weird the other day, it would not turn on, I unplugged the PC, plugged it back in and it worked perfectly afterwards so I thought nothing of it.
I rigged the PSU up to another old board just to see if it was the PSU or the mainboard that had gone. Trouble is I only have an old board without CPU or RAM, and that won't tell me anything really. I tried rigging it up anyway, plugging USB devices and such in, as usually you will see LED's on those before powering on the system. Nothing.
My guess is PSU has blown.
Edit: About 2 years old, usage probably average for that type of hardware 4-5 hours a day... Maybe 3,000 hours on it in total.