A warm water tank is only really needed when the gas tank is remote from the loco boiler, i.e. in a tender, or in the back bunker in the case of the Accucraft NGG16. Most locos have their gas tank sufficiently close to the boiler that there is sufficient ambient heat to warm the tank and its contents, but without heating it so much that there is risk of an explosion.
DJB Engineering supply a very neat kit for allowing a steam/hot water supply to the NGG16 warm water tank. I find the best results are obtained by only having about 1/2 inch water in the tank, and just giving it a blast of steam every now and then, and that there is no need for the water to be much more then tepid.