I had the same issue with Southern water.
True story.
When we went to a meter I set up a DD with them and six months ago they wanted to just about double it because I had exceeded what I had paid them by a small amount. I phoned them and declined and said I would pay what I owed and keep the DD as it was. Nice but puzzled lady on the phone said that I couldn't do that, I had to pay their figure so I said "no I don't, I will pay you what I owe and cancel the DD, then I will set up a savings account and pay into that instead". "Blimey" she said "thats a good idea. I think I'll do the same."
The latest bill has just come in, it was less than I had in the water bill account and I have made a small amount of interest.
Its ridiculous. With both my gas and electric accounts, I set the amount of the DD online. The website warns me if I will owe them money at year end or if they will owe me a lot but I get to choose what I pay them.