We moved into our house in Sept 2004. Arranged for Powergen to supply us gas and electric at £50 per month. Soon after moving in got a letter from NPower saying they were our supplier. Phoned and told them they weren't, they said OK.
Occasionally we got statements from them saying we owed them "NIL". We phoned the first couple of times we got one and they said they'd update their records. After that we took to returning them "Not Your Customer". In March we decided to switch supplier to London Energy.
TODAY we get a bill from NPower saying we owe them over £500 for gas since Sept 2004!!! 
We have NEVER had a bill from them saying we owed them money, they have always been "NIL"
Powergen confirm they have been our supplier since Sept 2004
There is no way on God's green planet we can afford to pay ANYONE £500
What can I do? I've contacted Energywatch with the details and am waiting to hear back from them. I don't want them to send in the heavy mob or anything!