Okay well I haven't taken it out of the case yet, I unplugged all the SATA cables, the wireless card and the RAM and it still showed a red cpu light. It didn't boot up or post either.
That has to mean it's either the mobo, cpu itself, psu or possibly even the case, doesn't it?
I might've fixed on the cpu cooler wrong, but I doubt it.
I don't think the GPU is the problem because I got that second hand from someone who successfully used it watercooled for a couple of months.
I also managed to get a system speaker, so I've plugged that in for all future troubleshooting, and for all the times I've tried to boot it so far, it hasn't beeped once.