Thanks for the reply,
After fiddling with the settings I've managed to stop it from happening for now, though i'm still no wiser as to what's causing it. This is at 1920x1080 with everything set to max and all options on, which is what I had set before trying to change anything, so perhaps it's not any particular setting at all.
I'd managed to get it to stop happening before, in beta 2, without knowing exactly what I did to stop it either. It seems to come back at random, but what makes it difficult to work out is that the graphics preset option doesn't change what the other options are set to, despite it still having an effect on the graphics in game (unless i exit the game and restart). As well as this, whether I change the settings in the menu or in game seems to affect it differently.