Yup thanks, I'd just seen this and it works a treat. ( did search before posting btw, but I guess my terms weren't general enough )

I was just coming back to edit my post when I saw that you beat me to it

Thanks again.

I run Windows 8.1 with a dual monitor (extended displays) setup attached to a Radeon HD 7870. Normally (100% of all other games I play) when I choose full screen on a game, it is displayed on the monitor that has been set as the primary display. Factorio on the other hand chooses my other display ...