I've just had my third letter from Energywatch, acknowledging that Npower are not replying to my letters. There were three articles and an editorial in last Saturday's Times, exposing the situation.
Npower acknowledge that they charged more than 4572 at the higher rate, but only if you believe that a year consists of 365 days:
“What we promise is that we won’t charge gas customers more than 4,572 units at the higher rate in a tariff year, which is not always the same as a calendar year. In our short tariff year of seven months from April to November 2007 we didn’t charge anyone more than 4,572 units at the higher price, but over a full 12 months, we did."
No promise of a refund though.