pot pfl 50L same as geoff jan 09?

Same prob as Geoff Jan 09?.When the programmer calls,the boiler fan runs for 1sec,& does this for about 10/20mins without ignition,not even a spark. mine is getting longer between fan runs & ignition.If i break the air seal on the casing the fan runs continuously 5/10 mins & i can feal the air being pulled in,then ignition & continues normal running as per required when called for by the appropriate room/cylinder stats.As soon as it fires the first time i obviously re/seal the casing,WHICH IS ONLY OPEN SLIGHTLY,ie loosen retaining screws 2 turns,so retighten them.But when the programmer shuts the system down the prob starts over when the programmer calls for heat/water on the next cycle.Could this be the the micro/switch on the airflow pressure switch sticking,if so can the switch be changed on its own,or do i have to change the whole thing?