So I am left with the wrong tariff problem and have found Customer Service (energy specialists) and the Complaints Team (energy specialists) incapable of either understanding the problem or how to address it even if they do.
This was first raised with Customer Service in writing on 29th January 2023 and escalated to the Complaints Team, by e-mail and by phone, on 2nd February. The problem was that my tariff was indeterminate, whether it was Next Flex single or Next Flex E7 or Next Online v9 single or Next Online v9 E7. It varied by who I spoke to, the account dashboard, the system e-mails and my meter.
Trying to obtain a statement (to find out the tariff) did not succeed until 10th March when the tariff did prove to be one of the four possibilities, the one I was advised at the end of January by two energy specialists on the phone. They had been surprised by the system changing their action to renew Next Flex single to Next Flex E7, the system changing it to Next Flex single rate 2 readings. This was not only the wrong version of the tariff but the wrong tariff.
Following my telephone complaint on 2nd February my tariff was changed back to Next Online v9 fixed for another year, the dashboard showed it as single rate but the system e-mail showed it as E7 dual rate. This tariff and its duality remained until 10th March.
So we now know which tariff the system is billing to ... Next Flex two readings single rate, but shouldn't it be Next Online v9 fixed economy 7 until 2nd February 2024, that is the tariff (with rates) communicated to me on 2nd February 2023 in the 'Change of tariff' e-mail following the action by the energy specialist acting on my complaint.
Where do I go from here? The complaints procedure does not seem to provide a means to investigate a problem, with continuity, and with a reference.