Tony - you should call in a specialist boiler engineer - when you call him be sure to tell him the make and model.
When the boiler has a call for heat the first thing is that the fan runs.
Then a pressure switch senses the air pressure and next the Control Board opens the pilot-flame gas-valve and makes sparks
Accordingly the Pilot flame should light - there may be a viewing window.
Another sensor checks for sufficient heat from the pilot flame and this is the trigger for the main gas valve to open. Voila - the boiler has fired up.
When you call the engineer you might be able to say which of these things is happening and which not happening - this will give some indication of the possible fault.
Good luck..., March 2015
