Ok, feeling a bit dumb right now. After spending 3-4 hours searching through forums and trying to fix this, I hadn't actually tried physically swapping the leads. I'd assumed that the screen positions would swap but the magnified resolution issue would remain. I tried swapping them directly after posting and now my monitors are back to normal.
I hadn't swapped them prior to the problem occurring so i'm not sure what caused this, but its fixed now.