A fluid pressure actuated apparatus for use downhole in a borehole comprises means such as a piston 150 for producing a force in response to a fluid pressure differential between well annulus pressure and a pressure source (above disc 36 in valve 100) isolated from the well annulus pressure; and means such as piston 122 by which extra force is applied to the force producing means in response to elevation of the pressure in a region 132 previously isolated from well annulus pressure. The force producing means may be arranged to operate a valve (36). The force producing means can be arranged to move initially at a first speed and then to surge as the extra force is applied.