Thanks for the replies guys, still no luck with this unfortunately though, gutted.
All graphics card drivers are up to date and everything. Never had any problems running games on this rig before; Assassins Creed 4, Battlefield 4, Tomb Raider, all on high settings so this just seems very out of the ordinary.
I've uninstalled and re-installed numerous times and ran a Steam integrity check but still can't help feeling I'm missing some files or something. No sign of the config.lua file anywhere that others have mentioned :/