Could be the 3 port valve, not changing over to c/h or the electrical contacts are dirty. Stand by the 3 port valve & get someone else to switch c/h on. Listen for the motorised 3 port valve to change over (sometimes it has a lever on it that can be viewed moving from h/w to c/h). If the valve changes from h/w to c/h but boiler does not start then the auxilliary contacts on the 3 port valve are faulty (not passing 240v to boiler). 3 port valves cost about £35 from screwfix - next day delivery.
John L, January 2010
