Cheers mate. I did ring a IT expert who said he doubts it's the PSU. He said he didn't think that was the problem due to the fact I'm getting flickering lights on mouse and keyboard.
You would be right to doubt this, since it's just plain wrong! A faulty PSU can be the cause of nearly any observable symptom; whether certain lights flash or whether fans spin and so forth isn't indicative that the power supply is working properly, just that those devices are getting power. Sort of like when you have a brown-out, your incandescent light bulbs will still work, but a lot of other stuff might not. The various components in a machine have different tolerances as for how far from the norm they will work.
It might be possible that the power supply is faulty in that it gives PWR_GOOD for a few seconds, then stops. If a machine does not receive PWR_GOOD, the CPU will sit in a reset loop. If this is the case than the PSU is actually working exactly as intended; parts of it's insides may have been hooped by a surge as the power cut or something to that effect. Good quality power supplies are designed to sacrifice themselves in that case. How a "sacrificed" power supply acts depends on how it was damaged. Not all power supplies just stop working; your's still seems to do something, but something kicks in afterward to stop it from powering up properly.
It could also be the graphics card, too. Really the only way to find out what is wrong is to swap out various parts with known good components.
The only way to really test that though would be to swap in a known-good power supply.
