A downhole actuator (16) includes a piston (15) within a chamber (17) on which fluid can act on either side to hydraulically activate the actuator. A hydrostatic equilibrium device (18) and a valve (14) are mounted around the hydraulic actuator. With a radial port (28) of the hydrostatic equilibrium device closed and the valve closed to seal the string, pumping fluid through the string operates the actuator. Pressure overbalance prevents movement of the piston (15). The radial port (28) is opened to drain fluid from the string until equilibrium is reached with the annular volume and the piston (15) can be moved.