A heat exchange system for a high pressure tank for storing a fuel gas, comprising: a refuelling valve for receiving pressurised fuel from a fuel supply depot; a supply valve for supplying gas from the tank; an in situ heat exchange device (200, 71) mountable within the tank; a first external heat exchange device mountable externally to the tank, first (100, 200, 201, 3, EV20, 12) and second (EV21, GF4, 6, 202, 8, EV20, 12) circuits for the flow of the fuel gas, in use, through the in situ (200, 71) and external heat exchange devices, a circulation control system (V1, EV20, EV21) for regulating the flow circuits and the direction of flow of the fuel gas through the tank and the heat exchange devices (200, 71; 201; 202); and conduits (100, 6, 8) for the flow of gas in the first and second circuits into, through, and from the tank and into, through and from the in situ (200, 71) and external heat exchange devices; wherein the circulation control system (V1, EV20, EV21) is adapted to regulate: i. the circuits for the flow of the fuel gas within the conduits, ii. the flow and the direction of flow of the fuel gas within the conduits with respect to the tank, iii. the flow of the fuel gas within the in situ (200, 71) heat exchange device, and iv. the flow of the fuel gas within the heat exchange device mountable externally to the tank characterized in that the heat exchange system comprises a second external heat exchange device mountable externally to the tank and having an inlet connectable to the tank interior and an outlet, and an ejector pump having an inlet connected to the first external heat exchanger, an outlet leading through a one-way valve to the interior of the tank, when the heat exchange system is mounted to the tank, and a suction inlet connected to the outlet of the second external heat exchanger; and wherein the circulation control system is further adapted to regulate the flow of fuel gas within the second external heat exchange device; and in that the heat exchange system is operable in a first, refuelling, mode of operation of the tank such that: refuelling gas is led from the refuelling valve to the in situ heat exchange device (200, 71) to transfer the heat of compression from the fuel gas in the tank to the refuelling gas, and the refuelling gas then passes to the first external heat exchange device to transfer heat from the refuelling gas to the environment, after which the refuelling gas is delivered to the ejector pump, and then to the tank; and gas is drawn by the ejector pump from the outlet of the second external heat exchanger and is returned to the tank, while gas from within the tank is drawn to the inlet of the second external heat exchanger and passed through the second external heat exchanger to the outlet; and in that the heat exchange system is further operable in a second, supply, mode in which: gas is exhausted from the tank, wherein gas is removed from the tank and passed through the second external heat exchange device to the suction inlet of the ejector pump, from the inlet of the ejector pump to the first external heat exchange device and then through in situ heat exchange device for warming the fuel gas in the tank and is then led to the supply valve. a refuelling valve for receiving pressurised fuel from a fuel supply depot; a supply valve for supplying gas from the tank; an in situ heat exchange device (200, 71) mountable within the tank; a first external heat exchange device mountable externally to the tank, first (100, 200, 201, 3, EV20, 12) and second (EV21, GF4, 6, 202, 8, EV20, 12) circuits for the flow of the fuel gas, in use, through the in situ (200, 71) and external heat exchange devices, a circulation control system (V1, EV20, EV21) for regulating the flow circuits and the direction of flow of the fuel gas through the tank and the heat exchange devices (200, 71; 201; 202); and conduits (100, 6, 8) for the flow of gas in the first and second circuits into, through, and from the tank and into, through and from the in situ (200, 71) and external heat exchange devices; i. the circuits for the flow of the fuel gas within the conduits, ii. the flow and the direction of flow of the fuel gas within the conduits with respect to the tank, iii. the flow of the fuel gas within the in situ (200, 71) heat exchange device, and iv. the flow of the fuel gas within the heat exchange device mountable externally to the tank refuelling gas is led from the refuelling valve to the in situ heat exchange device (200, 71) to transfer the heat of compression from the fuel gas in the tank to the refuelling gas, and the refuelling gas then passes to the first external heat exchange device to transfer heat from the refuelling gas to the environment, after which the refuelling gas is delivered to the ejector pump, and then to the tank; and gas is drawn by the ejector pump from the outlet of the second external heat exchanger and is returned to the tank, while gas from within the tank is drawn to the inlet of the second external heat exchanger and passed through the second external heat exchanger to the outlet;