hi check your presure gage. it should be beteen 1.5 and 2 bar. yours is probably below 1.
you wil have to top system up. i think on the worcester you nee a tool. if not you should have a flexi hose with black tap on it. turn tap so water enrere the system. watch the rsure gage when it reaches 1.5 to 2 bar turn tap off. this should get your heating working again.
carl
August 2007