An actuator comprises an actuator output shaft, a rotatable lock formation associated with the output shaft so as to be axially fixed relative thereto, the lock formation defining an entry passage, a lock pocket and an exit passage, a pin positioned for movement relative to the lock formation such that, as the actuator approaches a fully extended position, a part of the pin is received within and passes along the entry passage, and resilient detent means operable such that, once the pin has moved beyond a predetermined position within the entry passage, the resilient detent means prevents return movement of the pin along the entry passage.