OK, quick update...
Just that on the last 4 shut-downs, I tried on:
Run 1, when I turned of my monitor, the PC stayed powered down.
Run 2, I left the monitor on, and the PC restarted.
Run 3, same as Run 1.
These were consecutive attempts at switching off, over a period of minutes.
Run 4, same as Run 1, after playing Call of Duty for 90 minutes, and browsing a couple of forums.
So maybe then, it is something to do with the monitor being switched on?