That was me that had that problem before. The first thing to check is that the fan is actually running, then check the air pressure switch. You can do this with a multimeter and check the switch terminals are actually switching live or not. I seem to remember it was 24v should be present at each terminal when the switch activates. If this is true, check that the voltage is getting all the way back to the PCB from the switch by tracing the wires back. If this is true, check the ignition is sparking. If this is true, then you might have a nasty problem
Also try removing the hose or hoses (depending on the age of the boiler, the later switches only have 1 hose) from the switch and check you can blow freely through them and that they have a good tight fit. It's also possible the fan is running at reduced speed, try releasing the burner cover screws slightly and you should hear a healthy sucking noise around the rim.
It could also be something a simple as a lack of pressure in the system, but then the pump warning light would be flashing. That's if the warning lights are not lying to you
There's a copy of the boiler manual online with a fault diagnosis, for what it's worth, here:
http://www.partsarena.com/baxi/System/DATA/Dx/DS1/installation/2288/I05-2288/I05...I hope it helps, good luck.
A new PCB is over £100 at trade, so it's best to eliminate all other possibilities first.