Went through a few more checks, seems it's the power supply. Took the existing Corsair TX650W out (which seems to power everything up) and placed in one from another machine, it booted fine, put the suspected faulty one in the machine that I just took the working one out of, that won't boot either. Strange that it powers fans, HDD, DVD and everything but doesn't give enough to POST.
Done voltage checks and everything seems to be correct on all the moxel/4pin/24pin connections.. Requested RMA from Corsair as it's 5yr warranty.
Thanks for help all.