the worcester 240 is a good boiler and it uses a diaphragm opperated change over valve which lasts about 5 year in a family home, it sound that this is your problem. you can just change the diaphragm but because of the boilers age i would change the compleat valve. How to diagnose, remove case and look for a brass valve with a black switch head on it. with help and a torch turn the hot tap on and the spindle should pop out and hit the micro switch contact causing the boiler to come on. If like your s the valve spindle doesnot move then replace the compleat valve.
andy the gas man, July 2008
