Jeff Layman brought next idea :
Out of interest, does your boiler's spec state that it is most efficient when
it is ticking over (or whatever it is doing to decide what heat it needs to
supply)?

So exactly how does the boiler do what it's supposed to do? You set the
boiler hot water temperature to the rads at, for example, 60 deg C, and you
set the room temperature thermostat to 20 deg C, and the hysteresis to 1 deg
C. How exactly does it (in the case of my W-B Greenstar) "operate with a low
flame if the demand for heat reduces"? How often does the "demand for heat"
reduce?

I had the boiler installed 14 months ago and I only recently added the
fancy controls. Until then it just had simple time clock and switching
stats controlling it. Then the boiler in operation was very noticeable
from both the noise from the boiler in operation and the ticking of
heating and cooling pipework.

Now the entire system is much quieter, because it knows it doesn't have
to run flat out, it knows how much heat input is needed so it can
modulate to run at a much lower level of burn. Before, it only
modulated down, based on its return temperature and often shut down
completely only to then light up again a few minutes later, because the
demand was still not satisfied. Rather like someone driving flat out,
then banging the brakes on, then flat out again - very wasteful of
fuel.