From time to time we may cancel and reissue previous bills we've sent to you. It means that we are amending your bill because we have received more up to date information. This will appear on your new bill as a Reversal.
This usually happens if we receive an actual meter read after an estimated bill has been issued and this actual meter read is lower than the estimated read used to calculate your previous bill.

You should submit regular meter reads. The more actual meter readings we have on your account the more accurate both your billing and any future estimates will be as we'll have a better record of your recent usage.