OK OP, buckle up, this is a long one.
I had a similar issue, not with Octopus, but with OVO energy recently. Out of nowhere back in 2021, they added more than £1500 in electricity charges to one bill, and said it was because we were on estimated readings and that the new reading had come through and updated the amount used.
However, we'd only had the account open 3 months at that point, and we'd paid our DD each month, based on their original estimation of likely usage when we switched to them, so an additional £1500 in charges over and above the £150 a month we'd been paying seemed unlikely.
I went through the account, downloaded every bill, and disputed the amount owed. There was absolutely no way we could have used what amounted to more than £2000 worth of electricity in 3 months. I spoke to an electrician friend who has done frequent work on my house and he confirmed that nothing we had in the house would be pulling that much power.
I disputed the bill with OVO who did some tests, meter creep, checked day v night readings, and then they insisted it was correct. It clearly wasn't. I had several arguments with them over it, over several months, and refused to pay the excess. Eventually I did a subject access request and got copies of every single meter reading and all the activity on the account. I went through them line by line, and it became clear that at some point the readings jumped around massively, at the same time as an OVO employee making some sort of correction on our account.
Anyway mid 2023 I complained to OVO formally, who did nothing with my complaint other than chase me for payment. I also reduced my direct debit to a very low amount, and when they kicked up a fuss, I told them I wasn't paying any more until they could prove the bill was accurate. That still didn't get any resolution. I had to wait 8 weeks from initiating formal complaint until I could go to the ombudsman, (Ombudsman rules) and in that time OVO did nothing useful.
So I took it to the ombudsman, who investigated and agreed with me that OVO had got it massively wrong. OVO accepted the findings and reworked the bill, they wrote off around £300 as back billing, and then insisted the bill was right, and that I should pay it. But it was still clearly massively wrong and OVO wouldn't give me any evidence of what the error actually was. By this point we were into 2024.
So I took it back to the Ombudsman again, who did a great impression of chocolate teapot. They did basically nothing to make OVO comply with their original finding. I put in a complaint to the Ombudsman and eventually got a 'soz, we got that wrong' response, but they didn't actually do anything useful.
I left it for a while, and then got fucked off with it when I started getting debt collection letters. For context, I previously worked in customer service in a trouble shooting type role for a long time, so I have an idea of how these types of investigations should go. I put together basically a huge dossier of exactly what I thought had happened, all the paperwork, a six page timeline of all my interactions with OVO, all the documents I had from them. I did the job for them pretty much. That thing was a bloody work of art.
I sent that dossier to OVO's CEO, my MP and every newspaper I could find that did a consumer issues piece. All copied into the same email. My MP was excellent, he was on them in a few days asking what the problem was and why weren't they sorting it. The CEO's complaints team also responded, and I got an actual assigned account handler who reviewed the whole account and actually for the love of god listened to me. Eventually they wrote off the errant £1500 and gave me about £300 in credit as compensation. They agreed something had gone very wrong, but couldn't tell what, so they just... wrote it off.
But from the issue appearing in 2021, to the resolution earlier this year, took 4 years. And if I hadn't been so bloody minded about it, I might never have got it solved.
Woooo that was long.