Water Bill

I have recently had the most curious dealings with my water company. The flat in which I live has a water meter. I’ve been there since August and my first statement came in January. They’d read the meter and I supplied them with a meter reading when I moved in.
For those four months, and including the annul standing order, my actual bill came to £48.00. Interestingly, my direct debit had been for about £12.00 per month, which seemed about right, although my total bill for the year will mean that I’m overpaying as I won’t pay another annual charge until next year.
For some strange reason, they estimated that my total bill for the year would come to £239.38. Where did they get that from? Maybe that’s what a flat this size normally does. But I’m not here at weekends, and sometimes not other times as my job takes me all over the place. Clearly, from four months’ usage, you can see that I do not use that amount of water. They suggested a monthly payment of £19.00.
I protested. They agreed to bring it down to its former level. They didn’t hesitate. I get the impression they knew they were trying to pull a fast one.
All fine and dandy. Except that on the new statement they tell me that my payments will not cover my charges. Pardon?
£48 / 4 = £12.00. I’m actually paying £12.61. What I should be paying is £48 -16= 32/4=£8.00 per month. I think at the end of the year they should be giving me money back. Should I demand the interest?
Nice try!