When the timer/programmer and either of the thermostats for room heating or hot water ask for heat the first thing to happen is the fan running - then the fan operates a pressure switch - then the pilot light lights - then the main burner fires up.
Switch it all off and switch it on again - make sure the programmer is calling for heat - listen for the fan running.
If not go to bed early and use Yellow pages to call out a boiler engineer.
Good luck...
Peccavi, February 2009