Hi Andy,
Just a thought but as a refinement to Colins suggestion why not use the rudder servo to turn the turret as well?. Obviously this would depend on where the rudder servo is fixed and may need a long linkage but it would mean you did not need a seperate servo and would also give a visual indication of the rudder position.
I’m glad to hear your Leander is nearly ready for the water. Did you manage to get hold of a Scout helicopter?
Grumpy