Not sure of your detail - if they have literally made up 💩, hopefully you will win…
….. but something I learned with Octopus some time ago was to NOT build up excessive credit with energy companies.
Their regulator will only allow them to ‘back bill’ up to 12 months…..BUT if you have credit in the account, they can take older usage (that they might have failed to bill due to their systems) from that credit 😳
We had several months spanning a period just over 12 months old that they had buggered up their readings for, and because we had some credit in the account, they are allowed to take for it.
Some folk will (rightly!) say “well, you used it, you should pay for it”….but the reality is that their systems are sometimes built on quicksand. If you do not leave (much) credit in the account, they cannot bill for energy used more than 12 months ago.
Energy companies are not banks 👀
Good luck!