This has finally been resolved.
Octopus have managed to retrieve all my missing consumption data starting in Sep 2023 to late Feb 2024. The information was there somewhere, possibly in the meter itself which was still flashing away while no data was coming through to Octopus. The calculations from the GivEnergy data was within 2% of the real bill.
In the meantime they issued various bills, using Flexible Octopus rates. Actual bills are around £200 less so I've got a hefty credit on the account, helped along by lots of export recently.
Next decision is whether to go for the Agile tariff. My average cost per kWh imported is under 19p in winter, and 15p in summer - and will be dropping again in July. I'm using Wonder Watt to export from 17:00 and normally discharge 3.5kWh from the battery, earning about 30p every day if just charging the battery overnight in the cheap period.
I get export from solar as well - about 20kWh yesterday, best ever day with nearly 35kWh generated.
Would I do better on Agile?