Question Author
Thanks everyone.
Incidentally Eddie, modern houses have mains water to the bathroom cold tap, so it is OK to drink from them now. The cold tank in more recent times only feeds the hot system.
I asked the question because a couple of houses I have lived in have had open tanks. I have tried to fashion covers out of hardboard and cling film, but wondered if I was doing the wrong thing.
I will look into buying a proper cover for my current abode, though haven't seen them in the stores.