According to my contract with npower, my gas consumption for the first 4572 units per annum will be chared at the primary unit rate, which is 6.88p per unit now, and those above 4572 units will be charges at secondary unit rate, 1.999 per unit now.
However, within the last two years from 05 March 2006, the day I started unsing npower, to 14 April 2008, my last bill day, Npower charged me 12,502 units at the primary rate. While the correct units to be charged with the primary rate should be 9,670 units calculated by the days past. That means 2,832
units I consumed was over-charged by npower with higher price. After calculation, I was over-charged for ?113.52 by npower in last two years.

I have wriiten three complaint mails to npower, but was unable to get the reasonable response from them. What can I do now? or where else I can complaint to.