scottish power were my previosu supplier and thier bills seemed to cover varying periods, not just the usual quarter or 13 weeks, if you look at previous bills you may find that they haven't sent you a bill that ovrlaps they just didn't send a bill for the right weeks last time, because they send a bill after a meter reading rather than every quarter on the dot sme time each quarter, really annoying it is. the gas was only read once a year too! rest of the time it was estimates.