There are some on here who will scold you for interrupting the gas supply without certification - but not me.
The manual has a fault finding chart in the back.
The sequence is...
Call for heat (programmer & thermostat)
Fan runs
Pressure switch senses fan running and signals circuit board
Circuit board opens pilot valve and makes sparks
Circuit board senses pilot alight and opens main gas valve.
Your fault is associated with the pressure switch or the pilot gas valve or the control PCB. Many people suspect the PCB of having bad joints and here's a link that shows how to remove it - pictures & text.
http://kaijaks.co.uk/blog/nick/2008/02/10/dodgy-old-boiler
Good luck...
Peccavi, January 2009
