Connected to the wrong meter

So as the title suggests, after about 5 months of being with octopus and gas usage looking normal, at month 4 we noticed the usage going way up without changing anything we were doing. So contacted octopus and they got us to do a burns test to confirm they were connected to the right meter for our flat (live in a 9 flat complex). So it turns out they were connected to the wrong one so we have been paying for another flats usage. Apparently our actual meter is currently connected to British gas so I guess someone else has been paying for that. Now I've had response saying that once everything has been switched they can "learn our energy consumption" and "back date estimated meter readings" to re bill our account. Surely there is data on the smart meters that dates back to when we joined octopus to now so they can find out or real usage not estimated and then compare this to what we've been billed for and then adjust the amount?