Got to my PC an hour ago, turned my PC on, only one monitor came on, brilliant.
Restarted my PC, then both stopped working. It's taken almost an hour but I've finally got them back on again (disabled DisplayPort 1.2 and DDC/CI and pulled the power out of each monitor for a few minutes).
Honestly I'm gobsmacked that things can be this difficult, I really am. I'm reluctant to even try the EDID stuff as I fear I'll only end up even more stuck than I just have been. I think I'll be purchasing a couple of MiniDP to HDMI cables and seeing how things work with that...