You only need to have your boiler on its highest setting in the depths of winter. Otherwise, it is needlessly heating water to a very high temperature and wasting energy.
Your boiler is usually fitted with two thermostats - one for heating and the other for hot water. The heating thermostat is usually identified with a radiator symbol and has numbers marked or an increasing scale from minimum to maximum. This sets the temperature of the water supplied from the combi boiler and pumped through the radiators. Adjusting the thermostat to reduce the flow temperature through your radiators can save energy.
Your boiler will work safely when a lower temperature is set but you may find that your house is too cold if the temperature is set too low, especially if the external temperature is also low. You may find you need to adjust the flow temperature up on the combi boiler when the temperature outside is low then back down again when it is warmer.
Using your heating timer will make sure you are only heating your home when you need to. For example, in the winter you may wish to heat your home before you wake up and come home from work. Leaving your heating on through the night or when you are not home will waste energy and increase your bills.