It is very hard to set up heating controls when the heat is not required, and the reverse is also true. One can guess what is likely to happen, but one small error and the result is not what one expects.
I tried to set up my beer brewing in the winter, in an old fridge/freezer with a small heater, and a STC-1000 controller, I was sure I would not need cooling until the Summer months, however over heating started to become a problem from March onward, I had missed one little thing, fermenting gives out heat.
With the heating of the house again one little point over looked, the bay window can cause the room to get very warm in the afternoon. In the Summer the sun is higher and not so much a problem.
It does raise questions on if a bay window is really a good idea? OK an conservatory you can shut the inner doors, open vents and simply not use when too hot, but no doors between bay window and main living room, I am sure K glass would work, but it would need to be fitted the wrong way around. From when house was built we have removed the open grates and the through draft that went with it. So the overheating of the room is a modern problem. What is lacking is information.
If the manufacturers stated with their TRV will keep the room to +/- 3°C of set point, and another stated +/- 5°C of set point you would have some idea. It it said it has two sensors one on hot water and one in the air to compensate for the heat from radiator again some idea of what it's doing. However it seems all we can do is suck it and see.