Don't have the answer just similar problem. It has happened 4 times now. Just to explain:- Hot water OK but Central Heating does not come on when stat calls. Sometimes it starts working again for a day or so then stops. No faults indicated on Boiler. Replaced Stat and engineers have been out 4 times. They think they have fixed it but a month later same thing happens. Seems there could be a design fault with these boilers.
Peter, January 2008