A downhole hydraulic actuator tool (10,110,210) for use in conjunction with a variety of downhole tools to be controlled, said downhole hydraulic actuator tool configured to engage and control actuation of a downhole tool or device to be controlled, the downhole hydraulic actuator tool comprising: a body (14,114,214) configured to be coupled to a body of an associated tool or device to be controlled, the downhole hydraulic actuator tool and the associated downhole tool or device to be controlled together forming part of a tubular string for running into a bore; an actuation member (30,130,230) disposed in and carried by the body of said downhole hydraulic actuator tool and axially moveable relative to the body, the actuation member comprising a tubular mandrel having an axial throughbore therethrough, the actuation member configured for attachment to, or to engage, an actuation mechanism of said associated tool or device to be controlled; an actuation arrangement comprising a piston configured to move the actuation member (30,130,230) relative to the body (14,114,214) of the downhole hydraulic actuator tool (10,110,210), wherein the piston is directly or operatively coupled to, or forms part of, the actuation member (30,130,230); wherein the downhole hydraulic actuator tool is configurable between an axially retracted configuration and an axially extended configuration, the downhole hydraulic actuator tool configured so that axial movement of the actuation member relative to the body of the downhole hydraulic actuator tool reconfigures the downhole hydraulic actuator tool from the retracted configuration to the extended configuration and applies an axial push force to the actuation mechanism of said associated tool or device and wherein axial movement of the actuation member relative to the body of said downhole hydraulic actuator tool reconfigures the downhole hydraulic actuator tool from the extended configuration to the retracted configuration and applies an axial pull force to the actuation mechanism of said associated tool or device; and a lock configured in a first configuration to permit movement of the actuation member (30,130,230) by the actuation arrangement and configured in a second configuration to provide a fluid lock across the actuation member (30, 130, 230) which prevents movement of the actuation member by the actuation arrangement. a body (14,114,214) configured to be coupled to a body of an associated tool or device to be controlled, the downhole hydraulic actuator tool and the associated downhole tool or device to be controlled together forming part of a tubular string for running into a bore; an actuation member (30,130,230) disposed in and carried by the body of said downhole hydraulic actuator tool and axially moveable relative to the body, the actuation member comprising a tubular mandrel having an axial throughbore therethrough, the actuation member configured for attachment to, or to engage, an actuation mechanism of said associated tool or device to be controlled; an actuation arrangement comprising a piston configured to move the actuation member (30,130,230) relative to the body (14,114,214) of the downhole hydraulic actuator tool (10,110,210), wherein the piston is directly or operatively coupled to, or forms part of, the actuation member (30,130,230); wherein the downhole hydraulic actuator tool is configurable between an axially retracted configuration and an axially extended configuration, the downhole hydraulic actuator tool configured so that axial movement of the actuation member relative to the body of the downhole hydraulic actuator tool reconfigures the downhole hydraulic actuator tool from the retracted configuration to the extended configuration and applies an axial push force to the actuation mechanism of said associated tool or device and wherein axial movement of the actuation member relative to the body of said downhole hydraulic actuator tool reconfigures the downhole hydraulic actuator tool from the extended configuration to the retracted configuration and applies an axial pull force to the actuation mechanism of said associated tool or device; and a lock configured in a first configuration to permit movement of the actuation member (30,130,230) by the actuation arrangement and configured in a second configuration to provide a fluid lock across the actuation member (30, 130, 230) which prevents movement of the actuation member by the actuation arrangement.