I have had problems with nPower (still my supplier) in the past with readings I gave them when I switched to them. Shan't bore you with the details, but suffice to say I'm pretty dogged in these things and kept emailing and phoning them, and even wrote to their head office. Eventually, they did acknowledge their mistake and took £50 off my next bill - this has happened twice.
I can only suggest keep going, keep a written note of everything and the name of everyone you speak to, and don't waste money calling them on anything other than their Freephone number: 0800 073 3000
Don't give up! Good luck