Hi Linda , the only time a boiler should kick in on its own is when their is a severe frost and a built in setting that you cant control makes it come on automatically.
My late mum 97 got confused a lot with how her heating worked , NOT SAYING YOUR CONFUSED , yes you have thermostat but depending on your boiler you should also have separate controls for water and heating on the panel of your boiler that also decide on how hot water and radiators work. So you could have a situation where the boiler water and heater is independently set too high and the end result could be your water is scolding hot and your radiators are red hot even with the valve set on only No.2
I have looked on google at your thermostat , it is a portable one yes ! This mean it will respond to the room it is in. so if you put it in a cold room it will not stop working until THAT room is as hot as the thermostat wants it to be. The downside is all other rooms could be too hot.
Put the thermostat either if the hallway or preferably the room you use the most, when that room is warm enough the thermostat will cut out , if the other rooms are not warm enough just turn up the individual radiator valves.