The ETX extension arrived and I could tuck it neatly underneath the motherboard tray. The only thing I was missing now as a way of turning it on, seeing as the front panel connectors weren’t connected.
The PowerMac front connector used 1 cable with 18 pins that plugged into the mac motherboard to power the firewire, audio, USB, power button and LED connectors. This was no good for ATX, so I came across this thread (http://www.insanelymac.com/forum/index.php?showtopic=222735) where a guy in Spain hand made the cables suitable for ATX motherboards.
At €30 it seemed quite expensive and I was slightly dubious sending this amount of money to someone on a forum in another country, but when it the cable arrived I was impressed. It was braided beautifully and worked perfectly.