I have just moved into a property which has just had central heating put in?

i have a Drayton digistat +2rf central heating system however i have set the thermostat to 17c however the tempreture on the thermostat reads 21c and there is no flame on the screen so it is not calling for heat however the radiator is red hot, surely if i set a tempreture of 17c and the tempreture reads 21c the radiator should not be on until the desired tempreture is reached i have read the manual and am not doing anything wrong has anyone got any suggestions. please