Tried a few different BIOS settings today, nothing major. Nothing changed anything, so I put 'em back. Then I remembered this switch on the board which enables the TPU - I'd turned that off during earlier woes but the original owner had it enabled originally. Anyway, since it effects overclocking I figured why not try and enable it, the temps are all nice and low it might even solve the RAM thing.
It wouldn't even get to BIOS. Erk! I set it to the middle setting, and again no good. I turned it back off, and I still can't get the fucker to boot. Fucking hell, computers. Resetting the CMOS via the switch didn't help either, it would get part way through the boot and then seem to hang, but not always in the same place or with the same Q-code. So I pulled the battery for half an hour and then fired it back up.
It got to BIOS. More weirdly still, it shows 32GB RAM!
I enabled the XMP setting again since the reset had turned that off. It still shows 32GB RAM! And in Windows!
I think that probably means DSMITH is the prize winner, it's just a shame I solved it myself before I read that :)