Thanks for all the suggestions, but afraid I'm still haven't had any luck. Tried using a different PSU but there was no difference.
I also upgraded the ram to 800mhz to comply with the motherboard spec, but this didn't fix the problem (probably still worth me doing though).
Just to clarify, when the power switch is pressed the front LED flashes and the front fan usually rotates briefly. This suggests that power is running through but not powering up the motherboard.
The psu is connected correctly to the motherboard through both the 2x2 12V Power Connector and the 2x12 Main Power Connector.
I've scanned in a page from the motherboard manual if this helps at all. If anybody think any other pages would help them see the problem I'll scan those in too.