Yes, new PSU - and the cpu fan spins so I'm pretty sure it's not a power issue.
Using a new mobo, psu and graphics card.
The things that are the same are the CPU, the memory, the case and the drives. I suppose it could be the memory but I'd have thought it wouldn't stop the PC from POSTing. If the new CPU doesn't fix it that's what I'll look at next.
And I did try a different screen too
Irritatingly the case doesn't have an internal speaker so I don't know if there are any beeps.
On the bright side, assuming I eventually get it fixed I've got myself pretty much a brand new PC