Thanks for the advice guys but I can't take the PSU back as I got it used off ebay, however, I have discovered a workaround that may be of some use to anyone else with similar problems.
I read on here that sometimes these things will boot after many cmos resets so I decided to make myself a clear cmos reset switch using a couple of simple on/off rocker switches and the cable for an internal speaker and just splitting the wire from the middle clear cmos pin. This meant I didn't have to lever out the battery or remove my graphics card everytime I wanted to clear cmos.
I cleared the cmos 8 times with no luck, on the 9th time however, it booted normally, problem then was that it wouldn't shut down so out came the power cord again, and then back to the original problem.
After many frustrating hours of trying I found that if you follow a certain procedure, it seems to work properly everytime.
1. Remove power cord, turn off normal cmos jumper switch and turn on clear cmos jumper switch.
2. Leave for at least 10 seconds.
3. Turn off clear cmos jumper switch.
4. Insert power cord and turn on PSU.
5. Whilst holding down the power button on the case, turn on normal cmos jumper switch.
6. Hopefully, Robert is your mother's brother.
This method has worked for me 5 times now and hopefully others will be able to use it too.