npower nearly drove me off the ledge (or more correctly came close to been the final push at what was the worst moment in my life) because they kept fucking up their billing for nearly a year over and over again, the same shit.
In the end I got pissed and kept asking for more senior people til I got to some director of something or other, I explained what had happened, that I had all the evidence (bank statements showing I'd paid, their inability to meter coherently etc) and that my next call was going to be a solicitor and ofgem if it wasn't resolved.
They zeroed the account (which worked in my favour somewhat since I had used some of it, came and got an accurate meter reading and apologised.
In the end I got pissed and kept asking for more senior people til I got to some director of something or other, I explained what had happened, that I had all the evidence (bank statements showing I'd paid, their inability to meter coherently etc) and that my next call was going to be a solicitor and ofgem if it wasn't resolved.
They zeroed the account (which worked in my favour somewhat since I had used some of it, came and got an accurate meter reading and apologised.
Turned out later I was caught up in this shitshow on their side: https://www.ofgem.gov.uk/publications/npower-pay-ps26m-faili...
So yes, keep everything is good advice.