Who could have predicted this?
Oh yes... anyone with any knowledge of the IT industry. I am the one of world's greatest sceptics of all things "smart" but the need to improve our home's heating setup was too great to ignore any longer and I "smartened" it at the end of last year.
Obviously all proprietary systems like Nest were out of the question, no components were permitted if they required any kind of proprietary app or 3rd party server to operate.
Our combi boiler is 25 years old and was run off a basic programmer/timer with only two on/off periods and separate controls for hot water and heating - no thermostat other than the basic wax ones on rads.
The old programmer was replaced by a Shelly relay, with two big physical buttons to turn on/off heating and hot water manually if desired (buttons have LEDs in them to indicate state, regardless of how it was triggered.)
The radiator valves were replaced with the inexpensive but decent Sonoff TRVZB valves; room temp/humidity sensing nearly all by the ubiquitous, dirt cheap and reliable LYWSD03MMC devices (flashed with a custom firmware to remove the need for any stupid proprietary stuff.)
All of the above is tied together by Home Assistant running the BetterThermostat addon which means it can be controlled by phone with the app, or any device with a web browser. A simple script turns on the boiler if one or more rooms needs heat and turns it off a few minutes after no room needs heat. The boiler runs far less than it ever did and yet every room in the house is at an appropriate temperature at all times - all of it is accessible from anywhere (through my own VPN) if required, but none of it requires any third party service and even if home assistant became inoperable for some reason we can still manually turn the heating on and off.