A damper unit comprising: a first chamber; a second chamber; a first piston dividing the first chamber into a first volume and a second volume, and a second piston dividing the second chamber into a third volume and a fourth volume, each piston being moveable along the length of the respective chamber to vary the size of the respective volumes; a first connection between the first volume and the exterior of the damper unit, the first connection being capable of providing coupling for fluid within the first chamber with apparatus external to the damper unit; a second connection between the second volume and the exterior of the damper unit, the second connection being capable of providing coupling for fluid within the first chamber with apparatus external to the damper unit; wherein the second chamber has a cross-sectional area, in a plane perpendicular to the length of the second chamber, that is smaller than the cross-sectional area, in a plane perpendicular to the length of the first chamber, of the first chamber so that the second chamber at least partially slides inside the first chamber and at least part of the first chamber can be located around the outside of the second chamber; the first piston is attached to the part of the second chamber that slides inside the first chamber; the position of the second piston is fixed relative to the first chamber as the second chamber slides relative to first chamber; and the length of the damper unit changes as the second chamber slides relative to the first chamber; characterised in that the damper unit comprises: a third connection between the third volume and the exterior of the damper unit, the third connection being capable of providing coupling for fluid within the second chamber with apparatus external to the damper unit; and a fourth connection between the fourth volume and the exterior of the damper unit, the fourth connection being capable of providing coupling for fluid within the second chamber with apparatus external to the damper unit; wherein at least the part of the second chamber that can slide inside the first chamber is bounded by a structure having a double wall and the third connection runs through the void between the double wall; and the third connection is connected to the third volume by an aperture in the inner wall of the structure at the end of the third volume farthest from the second piston. a first chamber; a second chamber; a first piston dividing the first chamber into a first volume and a second volume, and a second piston dividing the second chamber into a third volume and a fourth volume, each piston being moveable along the length of the respective chamber to vary the size of the respective volumes; a first connection between the first volume and the exterior of the damper unit, the first connection being capable of providing coupling for fluid within the first chamber with apparatus external to the damper unit; a second connection between the second volume and the exterior of the damper unit, the second connection being capable of providing coupling for fluid within the first chamber with apparatus external to the damper unit; wherein the second chamber has a cross-sectional area, in a plane perpendicular to the length of the second chamber, that is smaller than the cross-sectional area, in a plane perpendicular to the length of the first chamber, of the first chamber so that the second chamber at least partially slides inside the first chamber and at least part of the first chamber can be located around the outside of the second chamber; the first piston is attached to the part of the second chamber that slides inside the first chamber; the position of the second piston is fixed relative to the first chamber as the second chamber slides relative to first chamber; and the length of the damper unit changes as the second chamber slides relative to the first chamber; characterised in that the damper unit comprises: a third connection between the third volume and the exterior of the damper unit, the third connection being capable of providing coupling for fluid within the second chamber with apparatus external to the damper unit; and a fourth connection between the fourth volume and the exterior of the damper unit, the fourth connection being capable of providing coupling for fluid within the second chamber with apparatus external to the damper unit; wherein at least the part of the second chamber that can slide inside the first chamber is bounded by a structure having a double wall and the third connection runs through the void between the double wall; and the third connection is connected to the third volume by an aperture in the inner wall of the structure at the end of the third volume farthest from the second piston. a third connection between the third volume and the exterior of the damper unit, the third connection being capable of providing coupling for fluid within the second chamber with apparatus external to the damper unit; and a fourth connection between the fourth volume and the exterior of the damper unit, the fourth connection being capable of providing coupling for fluid within the second chamber with apparatus external to the damper unit; wherein at least the part of the second chamber that can slide inside the first chamber is bounded by a structure having a double wall and the third connection runs through the void between the double wall; and the third connection is connected to the third volume by an aperture in the inner wall of the structure at the end of the third volume farthest from the second piston.