Potterton 15/2 repeated fan lock out?

Boiler serviced Aug 11. Late sept boiler went into lock down and would not ignite. Fan replaced and all ok for a couple of weeks then boiler started into lockdown again. Fan lock out keeps happening (daily!!!) and sometimes will not re-set. My local eng'rs have been fantastic in coming out every time - but we are all getting frustrated. Has anyone any ideas as to what is causing all this aggro??? Boiler is 5yrs old.