how can I fix our potterton profile netaheat 30e boiler ?

Our potterton boiler is 18 years old has recently had a new fan installed however the problem of not starting persists.Each time our when our gas man visits it starts but then it continues not to start up .Some times the fan runs and boiler will spark light for a short while then cuts out but fan still runs,sometime the fan does not start at all.Could any one help as we the temperature is only 11c is it the PCB?Any other suggestions would be gratefully received.