A similar thing happened to me with my electric meter. The electric company "didn't realise" that my meter had 2 readings (day & night ) for over 10 years ! I didn't know I was on economy 7 either! When they finally realised , I got a huge bill of over £1,000 which I refused to pay.I went to the CAB who are excellent, and one of their advisors wrote a letter for me to the electric co. The final outcome was that the electric board "wrote off" the outstanding amount in full , as they admitted it was their mistake ! Result !If you have always payed your bill in good faith, I would say that you have a good case not to pay any more. You dont supply & maintain the monitoring equipment, they do! Good luck.