Small thing that turned into a big thing: I always have the 'player 2' targeting stuff visible. Why is that? Oh right, the steering wheel I have plugged in for racing games is being detected as a secondary input. No worries, I can disable player 2's controller entirely in the options menu.

PROBLEM 1: Now it think I'm holding 'down' on my Xbox controller at all times.

Oh well, I've had problems like this before. Rebooting the game usually sorts it out.

PROBLEM 2: Rebooting the game doesn't sort it out. The main menu works OK but once I load my quick save it all breaks again.

OK well, a PC reboot tends to fix these issues when a game reboot does not.

PROBLEM 3: My PC no longer boots past the bios.

WHAT...THE...HELL. Half an hour of panic, faffing about and resetting BIOS settings later, I manage to get back into Windows. I boot the game... AND IT'S STILL BORKED. So I go into the options and re-enable the steering wheel for player 2.

Everything is fine now.