I have found exactly the same happened to me. All billing OK to the end of 2013 and then the next bill had swapped the day and night meter reading round - with the inevitable result that I am asked to pay more. I have spoken to Scottish Power's so called Customer Service department 3 times and have sent 2 email complaints with details of the error and all the back-up using the bills that Scottish Power produce. Result - I am ignored and all promised response dates are missed. My guess is that this is some problem in Scottish Powers billing system and that there are lots of people out there who are in for a nasty shock as Scottish Power increase their monthly DD amounts. I am still in the process of chasing this with Scottish Power, and needless to say have lost most faith in them!!! People who have Economy 7 should check their bills and the meter reading entry page as they may, like mine, be wrong.!!!!!