When I start the system without the GPU plugged into it boots correctly. If I plug the GPU in, all the fans appear to spin up however I get no display from either of the DVI ports on the graphics card. I don't think there should be any issues with the amount of power available, but I'm at a loss as to the issue. BIOS setting are default (i.e. no overclocking, until everything is working together).
With the GPU removed, output from the onboard VGA is fine. With the GPU plugged in there is no output from the onboard VGA (even if I enable onboard graphic 'Always' in the BIOS).
While I was checking that, I also pinged the machine while it is booted up without any display (i.e. GPU plugged in). It responds to ping fine. I think this rules out any power issues, so I'm trying to work out if it is a misconfiguration somewhere or a faulty part.
I confirmed that the machine was booting by VNCing into it from my laptop. I also tried plugging into the onboard DVI rather than VGA (which gave no output). As such, I concluded that there may be an issue with the monitor. Plugging into the graphic card through a DVI -> VGA converter works correctly (although doesn't look as good as I would like) - seems the issue was the monitor. Thanks for all your help above