My theory is that because the water (in the system) is actually
already at temperature and the house never goes below 20 degrees and
that this is a more efficient way to heat my house and keep the bills
down.

No, only in pathological lab setups will you find a system that is more
efficient if left on. It is far more efficient to turn it massively down
when not around. Any potential inefficiency will be due to overshoot as the
house warms back up. By choosing a sophisticated programmable room
thermostat, this can be avoided. This is because it will learn the response
of your house to heating demands and turn the heating back on just the right
amount of time before you are due to come home. This timing may vary
depending on the temperature.

When the room thermostat is not calling for heat the boiler
only comes on every now and then for approx 30 seconds, I assume just
to keep the water which flows through the HW cylinder at the required
temp.

Your system should always be pumping hot water through the cylinder. The
cylinder should have a thermostat that only asks for heat when the cylinder
is cold. This should cause a boiler interlock that prevents the boiler
firing when no heat is required. Keeping the primary circuit hot all the
time is very wasteful.

If you have a Y or S plan system, it is easy to add the thermostat. If you
have gravity circulated hot water, then you need to add a zone valve into
the pipework and control this with the thermostat after checking that it
won't damage the boiler. Some back boilers, for example, need to have the
gravity circuit open to avoid boiling.

If you have gravity circulated water, you usually have four water pipes from
the boiler. 2 will be 22mm for the radiators. 2 will be 28mm for the
cylinder. Obviously, combis will also have four water pipes, but usually
22mm for the radiators and 15mm for the water.

Christian.