taken from your boiler manul:
EFFICIENT HEATING SYSTEM OPERATION
The boiler is a high efficiency, condensing appliance which will
automatically adjust its output to match the demand for heat.
Therefore gas consumption is reduced as the heat demand is
reduced.
The boiler condenses water from the flue gases when
operating most efficiently. To operate your boiler efficiently
(using less gas) turn the central heating temperature knob
(A) to the ‘ e‘ position or lower. In winter periods it may be
necessary to turn the knob towards the ‘MAX’ position to meet
heating requirements. This will depend on the house and
radiators used.
Reducing the room thermostat setting by 1ºC can reduce gas
consumption by up to 10%