Check your energy bill to see if the reading type says "estimated" or "E" - if so, you need to submit your meter readings more often or get a smart meter installed to avoid issues from estimated bills

P.S. Our sensible reminder that deals are only deals if you were going to buy the item anyway. It's no use buying an item you don't need just because it's on sale - chances are, it was the same price a few months ago and it'll be the same price again in the January (or Spring, or Summer) sales.