dennis@home explained :
That's not a stat problem that's too much heat capacity in the radiators.

The stat stops the flow but the stuff in the rad is too hot.

You need to have low water content rads.

I agree, it was originally installed when the place had a much higher
heat loss. Single glazed, no CWI, not much insulation in the roof and
quite air leaky. But its a cost v benefit v need. Only one rad has
needed to be changed and that swapped for one half the capacity, in the
hall. The original boiler was around 32Kw, the new one 18Kw. The guy I
got to install it, even suggested that was probably borderline too big,
but I specced it.

Too much capacity has the advantage that - Nudge the stat up a couple
of degrees and it hits the selected new temperature very quickly. Then
once up to temperature, it barely ticks over.