An actuator is described which comprises a rotatable actuator member (14, 114) and a lock arrangement (28, 128) operable to lock the actuator member (14, 144) against rotation, the lock arrangement (28, 128) comprising an axially movable lock member (30, 130, 230a, 230b), the lock member being movable between a locked position in which it co-operates with a stop (34, 136) to resist rotation of the actuator member, and a released position, and an actuation device (62, 162) operable to move the lock member towards its released position.