We recently had a letter about using a unregistered drinking water and we had to get back to the Water Company within seven days or be fined.
It turns out that although we have been paying for water by direct debit ever since this facility was available and there has always been money in the bank to pay for this, a bill issued (but not received) prior to DD's being introduced, was outstanding. There had been no statements or reminders during the intervening period. The Water Company were not allowed to take the money because the mandate was issued after the due date for the bill - which was at least 3 years ago.
Something to look out for!