Hey - update. I got a mobo off eBay but got the same except it's the z77 version not the z77x.
Again, it's still start top start stop.
So I've replaced mobo, PSU, CMOS battery and no change. The only others, CPU, ram and GPU - could it be the CPU?
I've also tried building it outside the case. Also taken the ram sticks out and ran it with that, no change. Same with GPU. The CPU pins on the older mobo are all damaged to, but the CPU looks fine