A valve assembly (300, 400), comprising: a first fluid port (301, 401); a second fluid port (302, 402); a valve seat (303, 403) comprising an inner valve seat (304, 404) and an outer valve seat (305, 405), the inner valve seat being arranged within the outer valve seat; a moveable plunger (307, 407), having a body and being moveable between a closed position and an open position to vary a flow restriction at the valve seat, and arranged to simultaneously seal the inner valve seat and the outer valve seat when in the closed position; a first fluid pathway between the first fluid port and the second fluid port passing via the inner valve seat when the movable plunger is in the open position; a second fluid pathway between the first fluid port and the second fluid port passing via the outer valve seat when the movable plunger is in the open position; at least one fluid flow path through the body of the moveable plunger comprising an opening (309, 409) configured to form part of the first fluid pathway or the second fluid pathway, and wherein the cross-sectional area of the flow path through the moveable plunger is at least half of the cross-sectional area of the gap between the plunger and the respective inner or outer valve seat of the first or second fluid pathway of which the opening forms a flow path when the movable plunger is in the open position; characterized in that the valve assembly further comprises a moveable diaphragm (311, 411) for separating a portion of the moveable plunger from the flow path through the body of the moveable plunger; and pressure compensation means configured such that one of the first fluid port and the second fluid port, and a portion of the moveable plunger that is not in a direct flow path between the first and second fluid ports, are under substantially the same pressure when the moveable plunger is in the closed position. a first fluid port (301, 401); a second fluid port (302, 402); a valve seat (303, 403) comprising an inner valve seat (304, 404) and an outer valve seat (305, 405), the inner valve seat being arranged within the outer valve seat; a moveable plunger (307, 407), having a body and being moveable between a closed position and an open position to vary a flow restriction at the valve seat, and arranged to simultaneously seal the inner valve seat and the outer valve seat when in the closed position; a first fluid pathway between the first fluid port and the second fluid port passing via the inner valve seat when the movable plunger is in the open position; a second fluid pathway between the first fluid port and the second fluid port passing via the outer valve seat when the movable plunger is in the open position; at least one fluid flow path through the body of the moveable plunger comprising an opening (309, 409) configured to form part of the first fluid pathway or the second fluid pathway, and wherein the cross-sectional area of the flow path through the moveable plunger is at least half of the cross-sectional area of the gap between the plunger and the respective inner or outer valve seat of the first or second fluid pathway of which the opening forms a flow path when the movable plunger is in the open position; a moveable diaphragm (311, 411) for separating a portion of the moveable plunger from the flow path through the body of the moveable plunger; and pressure compensation means configured such that one of the first fluid port and the second fluid port, and a portion of the moveable plunger that is not in a direct flow path between the first and second fluid ports, are under substantially the same pressure when the moveable plunger is in the closed position.