is my Digistat RF1 faulty?

Our central heating pump did not work, but when i lowered the temperature on the digistat it appeared to cut in, i turned up the temperature and the pump/heating seemed to work ok for the rest of that day, The next morning the pump did not work again, so again i lowered the temperature on the stat, again the pump started working, has anyone else had this fault?, is it the stat thats faulty?