I left npower after I was sent an enormous bill.
It turned out that they hadn't read my meter properly for nearly a year and had ignored my meter readings, estimating bills over and over.
When I phoned them, on several occasions about this bill, I had patronizing women telling me that I had to stay at that address and give them daily meter readings for a week. Telling them that I couldn't do this due to health reasons, they kept trying to insist.
I asked them, if they would like to come and read my meter daily for a week, but they declined. They just kept on repeating, "We're just going round in circles, Mrs. ******". I ended up telling the woman that I was sick of the sound of her voice, put the phone down and immediately found another company to pay extortionate amounts of money to for my fuel.