an actuator for adjusting the length of an extensible member , the actuator comprising : a driving mechanism ; a rotary member arranged to be driven about its axis by the driving mechanism selectively in a first direction and a second direction ; and an elongate flexible member supported by a constraint means for transmitting force from the actuator to the extensible member in order to adjust the length thereof ; the elongate flexible member passing around and being engaged with the rotary member such that rotation of the rotary member in the first direction causes the elongate flexible member to be extended and rotation in the second direction causes the elongate flexible member to be retracted .