I would guess that your November reading is wrong. ESB Networks are notoriously poor in estimating bills, in every single case where I've had an estimated bill,there has been a gross under-estimation. I always correct this immediately and get an actual reading billed. They under-estimated your usage from August 18 to November 19 and you mis-tread the reading. Do you have actual bills with accurate readings from previous years for comparison? While the money part of the bills may vary with pricing changes, in the absence of appliance changes or consumption increases, the units should remain constant. HTH