OK, one more go. You pay for the water supply and you are perfectly entitled to charge another user that is supplied from your supply. This is a common arrangement - and I don't mean landlords charging out rented flats, I mean properties - flats or houses - which have been divided and now have several separate owners but one water supply. Ideally everybody would have their own meter during the conversion but it isn't always possible or the costs are prohibitive. The rules change if the other user is a customer of the water supply company so, as I said at the beginning, you'd have to check that. But you have already said the neighbour is not a customer. You just need to purchase and install a meter, tell them you have done so, and send them a bill every, say, 6 months. If they do not pay, they will be debtors to deal with in the normal way.