A positional adjustment mechanism comprises a movable plate having first and second positioning actuators and a biasing member to bias the plate against the actuators. The movable plate has three flat surfaces facing the actuators and biasing member one of which is not substantially perpendicular to either of the other two. The biasing member ensures that the plate is held in contact with the actuators during adjustment. Also, an adjustable mount comprises a support and a fixed bar extending therefrom, an axially movable bar extending from the support and being axially movable relative thereto, and an element movably mounted to the fixed bar and movably mounted to the movable bar, the first element being selectively fixable relative to the fixed bar and selectively fixable relative to the movable bar. This arrangement allows a great degree of freedom in adjusting the position of the element as it can be selectively fixed to either or both of the fixed bar and movable bar.