OVO - final balance on account

MrsBrush

We moved house on 18th October last year. We had a number of issues with OVO producing the final bill, and eventually I received a refund in December 2021, some 10 weeks after moving and after several rounds of emails and phone calls and a credit of £30 by way of apology for poor service. I was assured at this point that this was a full and final balance and the account had been totally settled and fully paid. It seems that there had been a technical problem and the billing wasn't working properly. As the final calculation was made and a refund agreed and paid, I thought no more of this.

I have had no further correspondence since, as I no longer have a supply from OVO, either at my old or my new address, I deleted the app several months ago, and made an assumption that there was nothing left to do. I have also not looked at the online portal since December 2021, as there was no need as they told me the account has been settled.
You can imagine my surprise when I received a phone call this morning from the collections team, demanding I pay an outstanding balance of £136.20. This is 8 months after I moved, and almost 6 months since they told me I had settled the final bill. 

I have found the meter readings which were taken on moving day within a few minutes of each other, which were then phoned through to OVO. It seems that somewhere in the depths of time and the mysterious workings of the billing systems they failed to log and register the final readings on the account that we sent through at the time. It now looks as though the account had not been properly closed off and therefore the refund should not have been made. It looks as though further statements were produced after I believed the account to be closed, and I have heard nothing, either by email, by post or by phone call until this morning. Having now looked through the statements on the portal that I assume have been produced since January, I still cannot see that the final readings were applied and the final estimated dates also look wrong.

What do I do? 

Apart from rant obviously :-(