but surly its output only and not input? and i would need to earth it too?
The +5 V pin is the input voltage OR output voltage. Huh ?
OK...When you connect a external voltage <--- USB or through the barrel, that +5 V is OUTPUT.
Whith nothing external connected, Well the +5 V is an INPUT. That is when you put a 4.5 V in ( 3 X AA or C or D ). A 4.5 V will work. I try it on a breadboard version.
And of course you must connect the negative to the GND of the Arduino, same of the battery controlling the servo motor.
FYI. I did some current measurement of a servo motor. I use two type of servo. One normal servo and one for continous turn. The current for a continious is : about 160 mA. and the normal servo : from 50 mA ( slowest ) to 200 mA ( fastest ) And my Arduino board with a 9 V battery, the current is about 40 mA. <-- That is no Led's, no shields, nothing connected except the control logic of the servo motor.
In your case, I am curious what type of current and voltage measurements ( at the barrel ) when you connect the 9 V battery.