Hm, it indeed appeared to be resolution related. I still run an unusual resolution of 1920x1200. I changed the Windows settings for screen resolution to 1920x1080...which the game "hated" as I couldn't actually do much so I had to alt-f4 out. After reverting to the native screen resolution, running the game again, faffing about with its graphics settings, I resolved the offset issue. It's as if the game hadn't properly detected the resolution when I first launched it, despite having run it from a fresh "installation"...
