NPower are the worst I've dealt with.
I'd been with them for a while, quarterly bills were always similar and based on actual meter readings and then they sent an estimated bill (based on their own estimated readings) that was about 1/3 the cost of any of the previous ones.
A few days later, before I'd even dealt with it, I got a letter from them saying that my current bill is so low they've concluded that I've tampered with the meter and so they're switching me to a "pay monthly in advance" service and threats of coming to inspect the meter + legal action.
Just how bad do their systems have to be to make an estimate that has no resemblance to any previous bill and then send a warning letter without looking into the circumstances of the bill?
Eventually got it sorted out, with no apology of course, then left them for SSE.
About 5 years later NPower were apparently judged by the regulator to have been misleading customers on pricing for years, as I got a cheque for about £80 as part of the agreed settlement.