Severn Trent Stupid Statement

I pay for my water by direct debit, this weekend my annual statement arrived, I'm in credit. I'll admit that £20 over 12 months is no big deal, but it is an overpayment. I expected a reduction next year or at least no change.
But Severn Trent accountancy is weird, my payments are set to go up by 20%!
Their reasoning, "You used less than expected this year, so you'll probably use more next year".
My response, " The direct debit will cease, in future I'll pay on demand for water that I've actually used".