Hi, I wish to set my heating to 7 degrees so that the boiler will fire up when the temperature drops below?

Set the timer to 'ON'. This way the boiler is now controlled by the thermostat 24 hours a day. Set the thermostat to 7 degrees and the heating will only come on at this temperature and below. The thermostat is going back to 18 degrees as it is showing you what temperature the room is now at, not what you have it set to come on at.