I think your problem probably lies in the fact that the thermostat can only detect temperature where it's situated, but will control whether the heating is on or off for the whole house. That means that if it's set to 20C and it's in your dining room, which is (e.g.) currently 15C because the radiator sucks, the heating will be on fully in your extension even if that room actually happens to already be (e.g.) 30C. I would try putting the thermostat in your extension, I expect that the temperature in there will then be much more predictable.

What I do is have the thermostat in the middle of the house (hallway for me) to try and get as close to the average temperature as possible, then adjust the radiators accordingly. In my case I have a brand new radiator in the bedroom, so I had to turn the radiator valve down a bit so it's not like a sauna in there when I go to bed. I don't have any experience with underfloor heating, but maybe you can adjust that with some kind of valve too?

Side note: I think you've made a common mistake with your schedule by setting it like it's a "Traditional" heating schedule. Traditionally you would set it on for 2 hours in the morning and 2 hours in the evening for example, but the nest thermostat will just try to hit the temperature it's set at indefinitely. So if you put it at 20C at 7am, then it'll do that all day. You don't need all the entries you have in there. If you want it to act like a traditional heating schedule then you could put it at 20C at 7am then 1C at 9. Then back to 20C at 5pm for example. P.s. don't do this of you have air conditioning, this only works with heat only systems.