The problem with estimated bills or direct debit payments, is that it can cause major concern if things are calculated wrong.
There have been quite a number of cases, were people have had checks done later, only to find that they have had an hefty bill to pay, because there was a mistake made. Yes some people might be lucky, and find that they may have paid to much, and are in credit. I recall one of the forum member's discussing this actual event, when he was involved in a similar incident, a year or so ago.
I personally had a large amount of problems, when converting from my previous supplier's to a dual fuel account with E.oN. All sorts of problems occured, including at one stage having two account's with different account numbers (so being charged twice). The main problem though was having estimated bills, when there was no need for this, because the electricity and gas meter's had actually been read by a meter reader a few days earlier, before the bill was sent out. Its a long story, and the final outcome was having E.oN install a 'smart meter', which both I and E.on can check together, for the dual fuel usage being provided. its made life a whole lot easier.