Well, that seems to settle it. I decided to go for a smart charge this evening as Caliban was down to 8% and I have to go to Glasgow tomorrow. I could just have charged for six hours using the Zappi's schedule, but I thought it was a painless chance to get a <10% to 100% charge done if possible.
I got home at 9.58 pm and plugged in, having asked for 100% by 6 am. I was given 9.58 to 6 am which was the general idea, and I don't quite see how they can change that. However, there was a new note on the app when the schedule appeared.
"FYI: Your charging plan may change over night, but don't worry we will still make sure your car is ready for when you need it."
The last email I had from Octopus said that once a charging plan was set up it shouldn't change, so someone hasn't got the memo. It looks as if this is a new thing, and certainly I never saw it before. I don't imagine they added that note just because I was moaning at them, so I suspect they have had a number of complaints. Odd that nobody on the customer service side seems to have been properly briefed about how to answer them.
Now pace John, but there are lots of people who have their systems wired so that the inverter will take power from the home battery if the car is charging, unless you take steps to stop it. It's perfectly easy to take these steps, but only if you know when the car is going to charge. Just set the home battery to charge during the car's charging slots, simples. Unless someone is then going to play silly buggers with the charging slots.
I predict trouble for Octopus here, as a significant slice of their customers (including me) switch off smart charging and just charge in the off-peak period using their cars' or their chargers' scheduling. If they only want one smart charge a month to keep the tariff that's not so hard to organise, but the rest of the time, forget it.
I imagine the other issue is with setting other household appliances to run when the car is charging during what would otherwise be a full-price time slot. How can you plan anything around these slots if you can't be sure they'll actually happen?
It was particularly annoying for me the first time this happened. As I had been given 11.00 as the start time for my charge, I set the home battery to export, timing the finish for 11 pm (rather than 11.30 as usual) so as not to have the car suck up charge that would otherwise earn a few coppers in export credit. Then when the car didn't start to charge at 11 pm after all, I'm left with an empty home battery and half an hour of peak-rate electricity still to go.
I don't think they've thought this through at all.