An actuator comprises an axially moveable piston coupled to a drive member such that axial movement of the piston 16 drives the drive member for angular movement, the drive member being coupled via a clutch to an output member, the output member further being coupled by way of a cam arrangement with an override actuator, the cam arrangement being operable such that initial angular movement of the override actuator relative to the output member can drive the output member for axial movement, disengaging the clutch, further movement of the override actuator driving the output member for angular movement.