mstanley - Thank you this is great and exactly what I was hoping for. Your thoughts on two motors pushing (albeit one less than the other) is easier to get my head around than what I had thought, I was trying to work it out with motors in reverse depending on position but I think that was where I was confusing PID input.
Essentially I am using an Adafruit motor shield to control the motors on my fixed point and the design looks something like below:
0 <------------------------------------------------------|--------------------------------------------> 160 (Fixed axis IR light)
Left Motor( O )Right Motor
So in explanation, as the IR light moves along its fixed axis the stationary object (represented by (O) ) turns to face it so the light is always the center of its "field of vision". I found a helpful explanation of PID control linked below but have been unable to make anything work.
About half way down, pseudo code post by scrat: