I've been asked to post about a recent experience with back billing and how it was resolved in case it was useful. Here goes...
We left eon in October 2021 and paid what we thought was a final bill.
In November 2022 we received another final bill, which came as a surprise over a year later.
It turns out that eon had only sent a final gas bill in October 2021, not a final electricity bill.
We were aware that we shouldn't pay for this as it comes under ofgem's back billing regulations. "You can’t be charged for gas or electricity used more than 12 months ago if you have not been correctly billed for it, or informed about it via a statement of account, before."
We rang eon. There was a long wait on the phone. Eventually it was answered by an inexperienced call handler who couldn't understand why we had been billed and wasn't sure if we should pay or not. They said they would have to talk to their manager and would ring back the next day. However, nobody phoned back.
We rang back a few days later and reached a different call handler. This time they said we were not eligible for back billing as we were now with a different energy provider. I told them that this was irrelevant, but they wouldn't budge. So they said they would send me a deadlock letter in the post so I could complain to the ombudsman. Two weeks later and I still haven't received that letter.
The payment was due, so we paid it as we didn't want to get bad credit. I then phoned eon a third time and reached a knowledgeable call handler.. They said that I should not have paid because of back billing regulations. They actioned a refund and resolved the complaint.
7 days later the refunded money reached my bank account.
If you get in a similar situation where you are surprisingly billed after more than 12 months, I suggest that you read up on back billing. And call eon several times if you think that your first call handler isn't dealing with the query knowledgeably.
Finally, if you google 'energy £30 rebate final bill' there are rules around getting £30 back if energy companies don't send you a final bill in a timely manner. We didn't complain about this as we just wanted to wash our hands of the whole thing!