Thanks all for your input.
Well I reinstalled the game, no overrides or mods.
Set in game settings to high except default 800 x 600 resolution.
I did a clean install of the 385.28 WHQL drivers (all default settings)
Mission still has a noticable black line.
In the Nvidia Control Panel, I usually just set the "Use my preference emphasising: Quality"
"Perform scaling on: GPU", "Override the scaling mode set by games and programs" and leave the rest alone.
I set the Global Settings "Anisotropic filtering 16x" , " Antialiasing - Mode Override any application settings" and "Antialiasing - Setting 8x" but it made no difference.
So I'll stick the mods back, try and forget about it, get back to retexturing everything, and maybe playing the game too.
EDIT:
Well it is definitely effected by resolution.
I just changed to 1600x1200 with everything stock and the line is gone. (well at least in this lighting where it has previously been there.)