Well it seems like my R3E, or my PSU has died. The motherboard lights up when connected to the PSU, and the PSU switches on, but the motherboard doesn't power on nor any of the fans connected to the board spin, but fans connected directly to the PSU work.

OCUK still had a load of Gigabyte X58a-ocs in stock, and for just ?139 so I ordered one with an XFX 850w xxx 80 plus silver PSU to diagnose. I'll keep the PSU regardless because despite the lower watts, its much better than my aging tagan and I don't even need my current PSU, just bought it because the price was good, and the Gigabyte motherboard is also a steal at that price.

Hopefully nothing else but either of those components is damaged, and I still have a few months warranty left on both.