Ok you haven't come back. I guess you haven't got a diagram so here is one possibility.
When your boiler fires up the flue gas fan starts, a differential pressure switch then closes if the fan is working properly.
It is not uncommon for the tubes which connect to the pressure switch to get clogged. This will give the sympton which you are experiencing.
Or the switch could be faulty.
You can test it by disconnecting the tube and gently blowing to operate the switch.
Alternatively it may be the fan which is not running at all.
So does the fan run then cutout.....ie fan ok
Check pressure switch.
ian, October 2011
