Turns out to be the PSU, the old one at 450W wasn't powerful enough to boot. Bought a 500W version and it works.
That however isn't the end of the story:-
Windows gets to the green scrolly bar thing right at the beginning and the system re-boots. This is constant. Safe Mode tells me to allow or stop "xmasbus.sys" and no matter what option I choose, the computer re-boots
Did a check online via my comp and this file has something to do with Alcohol-120 and a few people have had booting problems because of it.
xmasbus.sys is located in the folder C:\Windows\System32\drivers
Does anyone know how to delete this file without being able to get into Windows ?