A centre seeking actuator comprising: an outer cylinder comprising a first axial face and a second axial face connected by one or more first sidewalls to define an inner volume, the first axial face having a first through-hole; a slave cylinder slidably disposed within the inner volume of the outer cylinder, the slave cylinder comprising a third axial face and a fourth axial face connected by one or more second sidewalls to define a slave cylinder chamber, the third axial face having a second through-hole, the slave cylinder dividing the inner volume of the outer cylinder into a gas chamber and a working fluid primary chamber, the slave cylinder being constrained to move only between first and second extremities within the inner volume of the outer cylinder so as to leave at least a free portion of the gas chamber, the outer cylinder including a venting port in fluid communication with the free portion of the gas chamber; a piston-rod assembly (120, 122), the piston of which is slidably disposed within the slave cylinder chamber and the rod of which extends through the second and the first through-holes so as to project a free end from the outer cylinder; and one or more first dynamic seals arranged to act on an external surface of the rod to inhibit working fluid leaking from the outer cylinder; one or more second dynamic seals mounted on an inner surface of the one or more first sidewalls of the outer cylinder and arranged to act on an exterior surface of the one or more second sidewalls of the slave cylinder to inhibit working fluid leaking from the outer cylinder, wherein the one or more second dynamic seals are arranged to inhibit working fluid within the primary chamber passing to the free portion of the gas chamber. an outer cylinder comprising a first axial face and a second axial face connected by one or more first sidewalls to define an inner volume, the first axial face having a first through-hole; a slave cylinder slidably disposed within the inner volume of the outer cylinder, the slave cylinder comprising a third axial face and a fourth axial face connected by one or more second sidewalls to define a slave cylinder chamber, the third axial face having a second through-hole, the slave cylinder dividing the inner volume of the outer cylinder into a gas chamber and a working fluid primary chamber, the slave cylinder being constrained to move only between first and second extremities within the inner volume of the outer cylinder so as to leave at least a free portion of the gas chamber, the outer cylinder including a venting port in fluid communication with the free portion of the gas chamber; a piston-rod assembly (120, 122), the piston of which is slidably disposed within the slave cylinder chamber and the rod of which extends through the second and the first through-holes so as to project a free end from the outer cylinder; and one or more first dynamic seals arranged to act on an external surface of the rod to inhibit working fluid leaking from the outer cylinder; one or more second dynamic seals mounted on an inner surface of the one or more first sidewalls of the outer cylinder and arranged to act on an exterior surface of the one or more second sidewalls of the slave cylinder to inhibit working fluid leaking from the outer cylinder, wherein the one or more second dynamic seals are arranged to inhibit working fluid within the primary chamber passing to the free portion of the gas chamber.