This is Business as Usual for NPower
I was with them once, and I would never use them again nor wish them on anyone else. They are truly a showcase for incompetence which makes even the current government look good.
First, over a 2 year period they managed to bill me twice for the same supply, based on their "estimates" and they ended up owing me over £2000. It took 18 months to get them to admit their error and refund me.
A year later (why oh why did I not dump them then?) they send me a bill for £1,400 despite the fact I paid them by direct debit. This took 8 months to sort out, as I insisted on seeing a detailed breakdown of charges. It took a plethora of phone calls and 3 letters, (2 recorded delivery) before they saw fit to respond at all. However, their response was to instruct a firm of debt collectors. I contacted them again, meanwhile raising an official complaint to Energywatch, saying I had no objection to paying any sum owed as long as they could prove it was not yet another error, given the past history on the account. Their response was a letter stating flatly "the account is correct" and a letter from the debt collector saying they would sue me if I didn't pay. Again I wrote to them saying that them claiming that the account was correct didn't prove anything, and repeated for the umpteenth time my request for a detailed breakdown of charges. By now Energywatch had contacted them, and I received a letter from NP offering to knock £500 off the total bill. This seemed to me a pretty hefty amount for an account that "was correct".
Things however still dragged on, so eventually I said I'd see them in court, and they again upped their offer to wipe out most of the alleged debt, but still no breakdown. I didn't have time to mess about with these fools, so accepted the offer of settlement on the proviso that they close the account and never contact me again. They've already failed the latter, needless to say. I've had their telesales people on several times despite being ex-directory and registered with TPS.
AVOID NPower at all costs - life is too short. Atlantic energy get my vote however, they actually seem to understand something about customer service!