Looks like we need some water

With no shower, washing machine etc a tank full of water lasts us a long time. But I've just checked with my Chertsey patent gauge and it looks like we're running a little low. The gauge is just a length of clear hose. Attach it to the tap, hold the other end above the highest possible water level, turn on the tap, read off the level in the hose against the outside of the tank. Then turn off the tap (important to remember that bit), level the hose up, take it off and empty out the small amount of water in it. I invented that (although probably not for the first time)