A combination of a fuel tank assembly and a tool for opening an outlet of the fuel tank assembly. The assembly includes a tank for storing a liquid hydrocarbon; and an operator-openable valve arrangement configured to move between a closed flow condition in which flow through the valve arrangement is prevented and an open flow condition in which liquid is allowed to drain out of the tank. The tool is connectable to the valve arrangement, such that in a connected condition of the tool and the arrangement a liquid flow path exists between the interior and the exterior of the tank. The liquid flow path is at least partly defined by a conduit in the tool. A filter, in the flow path, includes a water-permeable member configured to enable water to pass through the water-permeable member and substantially prevent passage of liquid hydrocarbon.