I know this may sound silly but have you tried removing the cmos battery (power off) for a few seconds or a minute and replacing it, as this should set the BIOS to factory defaults... I always remove the Cmos battery to 'clear' its memory, even on a brand new board just to make sure it is fully defaulted bios settings.
Did you check the extra power connector? Or can you show a photo of the mainboard, at least the upper left corner of the board (the area i circled in my previous post)?
just for s**ts and giggles do you have another power supply you could try ? there pretty cheap if you go to a used parts computer store and it doesn't hurt to have an extra around if you have one bad lead it could be the cause of your problems !
Update. After trying everything suggested which amounted to over thirty different suggestions someone said to take the board out of the case and try it. First with only the power supply and monitor attached. It worked. So I put it back in and it didn't. Took it out again and it worked. Then I looked across the board from edge to edge. It appears to have a slight concave shape instead of being perfectly flat. So I suspected maybe the back of the board could be shorting out on the case. Longer standoffs would mean that the output sockets would miss the plate at the back so I just found some 3mm thick self adhesive rubber tape and stuck a few strips under the middle of the board. Put the board back in and it worked. Been running it now for about 48 hours with all bits and pieces installed and it works a treat. Thanks to all who have contributed. Especially Joe C in post #4 who nailed the solution.