Paul_D's post in Advice of wing mirror removal was marked as the answer
This video pretty much covers it.
There's basically just three bolts to undo from inside. Funnily enough, I did EXACTLY the same as the guy in the video. Managed to remove and replace all three bolts without dropping one...and then dropped the allen key into the door.
I ended up having to remove the door cards on both sides to retrieve dropped items in the end.