Thanks for the reply again. This is very useful.

Set Square wrote:

The setting is at about 9 o'clock on the dial which is pretty low.
Even then the boiler cycling is quite bad (off for about 40 seconds,
on for about two minutes continuously). When I set the thermostat
higher than this the cycling is even worse. It is the cycling that
has led me to consider taking action to get the system properly set
up.

Cycling occurs when the rate at which your boiler is producing heat is
greater than the rate at which the radiators and HW cylinder coil can
dissipate it. It should cycle *less* if you turn up the boiler stat because
with the radiators being hotter, they can dissipate more heat. Have you
tried increasing the pump speed?

I haven't tried messing with the pump speed (was concerned that this
might cause overflow into the header tank?) but I may experiment based
on your advice.

I will have a go at turning the thermostat up and measure the rate at
which cycling occurs. To be honest with you, I am not long moved into
the house and I'm still trying to find my feet regarding which radiators
to leave on or off while the system is running, and how long to run the
timer for. I'm in Northern Ireland and we pay stupendous amounts for gas
over here, so it's about trying to get a balance between heating the
place sufficiently and not getting an insane gas bill

Actually, re-reading what you wrote above - indicating that the boiler is
firing for 120 seconds in every 160 (i.e. 75% of the time) I wouldn't worry
about it if it only does it in steady state conditions. [I would expect the
boiler to run continuously for much longer than that when first starting up
from cold - does it?]

The boiler burns continuously for about 45 minutes from cold before it
enters the state where the cycling occurs. After this time I've got a
full cylinder of nicely hot water.