A resettable downhole tool actuator comprises a housing, a collet slidable within the housing and defining a seating with which a ball is engageable, in use, the collet being mounted upon a moveable support including a piston and arranged such that, when the bail engages the seating, fluid flow through an axially extending passage provided in the support is restricted, the housing including an increased diameter region arranged such that, when the seating is aligned with the increased diameter region, outward deflection of the collet permits the ball to pass through the seating and through the axially extending passage.