Take off radiator cap and check coolant level, it should be visible in the opening.
Otherwise may be an airlock in the heater circuit - you can try leaving heater on max temp and facing the front end uphill and revving it, the airlock may find its way into the main system and drop the level in your radiator a little, also check that expansion bottle is filled to correct level because if end of tube is not covered in liquid it can draw air into system as the engine cools down.
May also be the thermostat stuck open, pity that the Jazz doesn't have a proper temperature gauge so that you know what the engine temp is, as this would show a low reading. One way to check is to start car from cold and let it warm up while checking the radiator to see if it warms up too quickly (thermostat should block engine coolant from going into main radiator until engine (and therefore your heater) is warmed up properly, but allow coolant to flow through heater radiator loop).
