We had a new boiler and pump installed recently and the house was running at about 23C almost constantly. I was going around switching off radiators and trying to diagnose what was happening but getting nowhere.

Finally, today the answer was revealed:

At the very end of the central heating ring is a small radiator in the utility area of the kitchen; it doesn’t put out much heat of its own but it was still open 100% from the post-installation “powerwashing” which meant that the entire central heating ring was hot and feeding it all the time, so that the ring’s own pipework was acting like under floor heating and making the entire house toasty warm.

Lesson learned, and I can finally start meaningfully tuning the central heating, tomorrow.