Progress has been slow for lots of reasons. I now have reasonable designs for three device types:
Tariff Device
Purpose
For the import tariff (and, if present, an export tariff). Reflects the consumption for the day and for each price slot (where a price slot is shorter than a day).
Octopus Mini Device
Purpose
Reflects directly the data returned by the Octopus Mini on an instantaneous basis. Additional data is derived per hour and per day. This device could be used to implement the equivalent of an in-house display of energy use and cost.
Account Device
Purpose
The estimated financial state of the Octopus account. In practice mini readings for import differ slightly from “official” meter readings - consequently the estimated financial state will differ from the actual account balance. The actual balance is available from Octopus data, so it should be possible to continuously reconcile the estimated and actual balance.
I am unsure of the long-term value of the Tariff device since it only accumulates energy and value by slot (or day if the tariff is constant price). This data is also collected by the Mini and Account devices. Whatever, I plan to implement it and learn from its use.
Next Steps
Take the Google Script code emulating the devices and move it into a Homey app. Then lots of testing. Joy 
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Please Contribute
Any feedback or improvements on the designs posted here will be gratefully received.
