I'm going to try reinstalling, but I have used the key on Steam, installed the game, adjusted settings. When I "START" the loading screen moves through about 40% of its progress and then returns to the menu. I've tried changing settings to minimum, adjusting rights to folders, and run it with task manager open to watch that nothing is maxing out. It just stops.

Turns out the Steam Client being out of date was breaking it. I read the diagnostic files and they were very helpful. Well done, team.