A valve assembly comprising: inlet means (126, 128); outlet means; a valve body having a bore, and a valve element being received within the bore; a tubular sleeve insert being received in the end of the bore opposite the shoulder of the valve body; said valve element being arranged to control fluid flow between the inlet means (126, 128) and the outlet means, the valve element being moveable linearly between a rest position where it abuts the shoulder of the valve body and a fully-open position where it abuts an end surface of a sleeve insert; and wherein the valve element is moveable from the rest position into a closed position on application of a first control signal and towards the fully-open position on application of a second control signal, the magnitude of the second control signal being greater than the magnitude of the first control signal; and is biased to return to the rest position when the control signal is absent; and wherein: when the valve element is in the rest position, fluid flow between the inlet means (126, 128) and the outlet means is permitted at a relatively low rate; when the valve element is in the closed position, fluid flow between the inlet means (126, 128) and the outlet means is substantially prevented; and when the valve element is in the fully-open position, fluid flow between the inlet means (126, 128) and the outlet means is permitted at a relatively high rate; characterised in that the end surface of a sleeve insert defines an adjustable stop (116, 118) that defines the relative location of the valve element when in the fully-open position, with respect to the valve body. inlet means (126, 128); outlet means; a valve body having a bore, and a valve element being received within the bore; a tubular sleeve insert being received in the end of the bore opposite the shoulder of the valve body; said valve element being arranged to control fluid flow between the inlet means (126, 128) and the outlet means, the valve element being moveable linearly between a rest position where it abuts the shoulder of the valve body and a fully-open position where it abuts an end surface of a sleeve insert; and wherein the valve element is moveable from the rest position into a closed position on application of a first control signal and towards the fully-open position on application of a second control signal, the magnitude of the second control signal being greater than the magnitude of the first control signal; and is biased to return to the rest position when the control signal is absent; and wherein: when the valve element is in the rest position, fluid flow between the inlet means (126, 128) and the outlet means is permitted at a relatively low rate; when the valve element is in the closed position, fluid flow between the inlet means (126, 128) and the outlet means is substantially prevented; and when the valve element is in the fully-open position, fluid flow between the inlet means (126, 128) and the outlet means is permitted at a relatively high rate; when the valve element is in the rest position, fluid flow between the inlet means (126, 128) and the outlet means is permitted at a relatively low rate; when the valve element is in the closed position, fluid flow between the inlet means (126, 128) and the outlet means is substantially prevented; and when the valve element is in the fully-open position, fluid flow between the inlet means (126, 128) and the outlet means is permitted at a relatively high rate;