Octopus mini
Mine arrived this morning. Just set it up, which was painless. Not sure how useful it's likely to be if you have to get your phone out to look at the data (it doesn't seem to be available on the website, dunno if the API has access to it).
Inadvertent teardown, on account of the thing falling apart when I dropped it out of the box, reveals - unsurprisingly - an ESP32, a Zigbee wossname, some power regulation gubbins and not a lot else. Plenty of debugging/programming headers, which may get probed in due course.
ETA: It appears to be working. The instantaneous power figure agrees (to within the error expected from different sampling strategies) with my blinkylightmeter, and the half-hourly gas readings are there. The high-resolution electricity consumption graphs are rubbish though, seemingly implying that we're pulse-width modulating our electricity use so that everything is consumed in one minute out of every five. They also appear to be calibrated in Watt-hours per minute or something daft. Will give it a day or two and see if it sorts itself out.