hi can anybody help. i have a vokera option combi boiler that is over 6 years old. probably quite a few years older. the hot water is fine and fires up however the central heating wont work. it trys to fire up and does but shuts off after about 6 sec. it will continuously keeps trying but fails. ive fiddled about with the thermistor and it trys again to fire up but still no good. i can hear a noise build up for the few seconds after the burner has lit up but then it konks out. ????