Just an update on this, turns out the power supply is fine (ha Mitch), and it's the motherboard that has died. Thankfully my current motherboard is under warranty but it will probably take a few weeks to be returned which is longer than I'm willing to wait. So, I'm going to buy a new motherboard now, and when I get a new one from the RMA I'll sell it and only be out of pocket about £30 with a new working motherboard!