Fixed it, yay, but it was something so simple I should slap myself silly
.
The +12V connector wasn't seated properly...a capacitor was stopping it coming down fully. Noticed this morning when I was putting back together the old motherboard that it fitted fine the other way round as well...rebuilt new stuff on desk, popped it in the other way round and voila. Doh!!
At least now I have a £700 room heater/BF2-runner instead of a £700 silicon doorstop
Thanks for the help
P.S. I suppose I didn't connect the 4 pin supply.
At least, not properly. I thought I had it seated but as usual, a quick look instead of a proper check when running over my options in despair caught me out. Caught my mind this morning when I thought that the CPU fan was spinning a lot slower on the failed boot attempts. Ah well, live and learn :P