Is there a fix for the mouse controls? if anyone knows please shed some light if possible.
Sadly the best one I'm aware of is turning on auto aim in the game's launcher. It really changes how the game plays and the lock on feels a tad weird on PC at first but one can get used to it and it solves the problem with the imprecise mouse controls. At least it looks nice with the weapon automatically turning towards the target. ;P It's also how I played the game back in the day even though I never used auto aim in any other shooters at the time so I guess the mouse aiming was already wonky in the game back then and is not a result of modern specs etc..
A word of warning, though: the auto aim makes it practically impossible to hit a distant enemy who is moving horizontally on the screen with a projectile weapon. I always just considered this part of the gameplay, though. You can usually work around this with proper timing and maneuvering.
As for why the mouse is so weird: My guess
is that there's still some of the console version's code in there, e.g. the mouse only emulating analogue stick input or something and thus inheriting all sorts of behaviour that improved the controls on console but negatively affects mouse aiming - there's a number of PC ports of older console games that do shit like this and where the mouse aiming is just weird as a result.