well, now it's working - sorta. I've removed the ioboard from the motherboard but left the other wires going to it connected. Through a combination of unplugging the PSU, unplugging the 2 x 6 pin connectors, and flipping the switch, it does boot to the menu. I can't play games or access opsetup because the ioboard is disconnected, but I'm going to assume that works. I cannot get the thing to fire up with the ioboard connected. Thinking the ioboard has gone bad? Or maybe it's the PSU... I am going to try ordering an ATX to AT connector adapter and see what happens with a different psu.