How can I get my Digistat 3 to come on at set times ?

I had a Digistat 3 wireless installed in August 2006 together with a Worchester Greenstar 30Si combi boiler. In the last couple of weeks the thermostat will not come on or turn off at the programmed times. I have to turn in on manually by increasing the temperature on panel and off my decreasing temperature and then it works. I have replaced the batteries and display is showing as before. My plummer said the boiler is working and it is the thermostat which is the problem. Can you help?

Marianne
January 2010