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While doing some upgrading and tidying of my Home Assistant integrations and automations at the weekend, I discovered that my SonnenBatterie home storage battery now supports forced discharge via the API.
Which means I can now tell it to dump out most of its charge during peak export hours.
Sunday, I paid £1.44 for electricity (including standing charge), and exported £3.13 (battery and solar combined).
I'm also second-guessing my order for the EV charger - with the octopus agile go tariff, and a slow three-pin plug charger, I can get more cheap electricity slots, because the charge is slower than a 7kWh "grown up" charger, and the cheap rate applies to the entire house, not just the car 
@matstace You can limit the charging speed on a faster charger, so not a total loss. But perhaps no need to spend money on it then.
Said someone who put in a fixed 22 kW charger "for convenience".
@skolima that's true, although I don't know if Octopus will count that as "gaming the system" or not.
I ordered the 7kW charger when I bought the car, because "I have EV, I need a proper charger", and taking it from very low charge when I bought it to 80% using the granny charger took multiple nights and gave me the fear.
My usage patterns over the last three weeks though, make the granny charger more than enough, it seems.