After serious thinking harry wrote :
TRVs don't work. They are more influenced by the nearby radiator temperature
than the room air temp.
You can get them with remote sensors. They work.

They work absolutely fine here, once the heating has been on for a
while and the system has settled down. I am able to log/graph room
temperatures and once temperature is achieved, I only see variations of
around 0.5C in the graphs. What TRV's or any type of stat cannot cope
with is the overshoot, when the heating is suddenly turned on, or the
stat suddenly turned up to a much higher setting.