@Tirnanogg smart meters are a bit of a misnomer, I think; I got one installed when I lived at my old place and it was a total waste of time; the company wasn’t even receiving readings from it!
Octopus clearly has form for sending massive bills after switches and not telling customers of the dates switches have happened, to tell you they’re supplying your energy now. That information should also contain a request for an opening meter reading, because it’s that that triggers the opening bill, I think.
In absence of all that happening, I was racking up energy usage with them for months, I got busy over Christmas as you do, the app still showed me being in credit. I finally chased them in feb (switch happened in November after an initial request in AUGUST the day I moved), they asked me for a meter reading and then sent me a £1500 bill. When switch happened in November, they unilaterally moved me from the September 2020 tariff to November 2021 tariff; gas prices were twice as high and on the latter but I had to do major detective work on bills to work this all out for myself. Easily the worst switch ever.
Switching is supposed to be easy, and consumers should be able to handle it themselves, but in my experience it NEVER EVER IS.
Even before wholesale gas price increases, energy companies were so appallingly run - incomprehensible bills, poor switching, lack of communication. It’s a disgrace and it seems, ultimately, all companies are equally bad. Octopus is supposed to be a multiple Which award winner, FFS!