It is strange that it is a smart meter, but they send out readers. Is it dumb and not sending readings? Are you allowing the readers access?
If you are allowing them access for readings, I don't think you are to blame.
Have the company asked you to provide readings yourself? If you have refused and stayed quiet, this would be wrong.
As others have said, if they are not billing you correctly, through no fault of your own, they can only back bill 12 months.
I had a situation where I was faithfully providing readings every month, but the bills were always wrong, as they started at the wrong reading. I had evidence as it was a rental and my inventory had a photo of the reading, but their system wouldn't correct it.
I chased and chased but they dragged it out for 2 years, insisting that I owed them £9k for 2 years of gas! Eventually about 6 months after I moved out, they accepted a fair offer from me, as legally at that point they could only bill for the last 12 months. If they'd continued to drag it out for another 6 months, I'd have legally owed nothing as it would be beyond the 12 months. It was a particularly high bill as the dispute meant I was on their highest tariff and couldn't move or fix. I think I paid £600 in the end, rather than £9k.
They also supplied my electricity, which I of course paid, as it was correctly billed.