So got one of these with the extension kit in the end and installed it tonight. My setup is boiler, controller and ext thermostat. My install:
A few things I guess for those interested/on the fence:
1. This is super easy to install yourself, I hate electronics and all it took was a few hours. The extension box in particular was essentially plug and play - zero rewiring needed.
2. You need a PC to get the instructions, it wont work on phones at all. You essentially stick in all your setup and it gives you step - by step instructions on installing. You have to buy the product to get these as it wont give you access until you enter in serial codes etc...
3. You may need electrical tape, and a spare electrical box especially if your replacing a wireless thermostat (they say to put the old wireless unit back after making you do some re-wiring but its impossible to make it fit safely)
4. I got the v2 thermostat, and it works fine with Alexa - so not sure why they are pushing that only the v3 can.
5. The app is some confusing **** to start off with, even now I have no idea if its set up correct but we shall see.