Hi, thank you for responding. I understand about potential difference ect... I did not understand why the negative/ground of the Arduino would not be good enough as a reference point for the PWM signal it outputs. By making the reference point common I can see that what ever signal voltage is output by the Arduino will also be relative to the servo.
I'm trying to understand if there is something happening inside the servo. For instance, the square wave from the output from the Arduino should cause a similar current to flow and if so this flow of current travels down, in my case, the yellow wire gets to the servo and needs the servo ground to complete its circuit.
Would it work if I did not tie the grounds together but 5 volts coming out from the Arduino might be seen at the servo as something different, albeit slightly?
ARRRRRRRRRRRGGGGGGGGGHHHHHHHHHHH! I'm normally really articulated but even I'm laughing at my attempt at an explanation! I will common them up regardless but I'm the sort of person who likes to understand what is happening...