After talking to the supplier, I have tried the motherboard again with one stick of RAM, checked the heat sink is attached properly, checked the CPU is not damaged etc etc etc.
Then I tested it again with a minimal setting outside the case to check the board was not shorting on the case.
No joy.
Also checked the graphics card to ensure it was not damaged in the changeover.
Looks like I have a duff motherboard, which would be unfortunate for a first attempt at a PC build!
So I have rebuilt my old system and the mobo is going back to the supplier!
Thanks for all your suggestions.