Andrew, I've just had a look at your photos and I'm afraid I can see a bit of a challenge with your rudder linkage layout. As it is if the servo rotates clockwise the linkage to the port rudder tiller will tend to rotate about the connection point and the rudder will move very little. If the servo rotates anti-clockwise there will be more movement as it is effectively pulling on the linkage.
You need an arrangement where the movement of the rudder is approximately the same as the servo arm movement and the same on both sides. Something along the lines of this would be much better but the ratios of distances must all be as shown.
