bolier not firing up

Hello everyone. i\\\'ve got this problem with my boiler which has been going on for over a month. the problem is that it would not fire up when asked to do so, i\\\'ve had two plumbers in to look at the problem but they seem to be just as confused. they have replaced four air pressure switches, a pcb box, and a igniter. these prblems temporarily solve the problem but when boiler reaches its required temperature and shuts off, and then required to start again in dosen\\\'t want to know. at the moment when the boiler thermostsat is turned on the fan starts up but that is all that happens, there is no ticking sound to indicate the ingnition process. so the plumber has to blow into the red and clear pipes in the boiler for ingintion to start, he has checked almost everything from voltages to earthing and everything seems to check out except the voltage on the orange wire that goes to the air pressure switch. please can you help

It would appear you know the possible cause of the problem.
The 'fan' creates a vacuum and through a tube moves a diaphram which in turn allows a microswitch to make contact and allows electric to go to the boiler to operate it.
Of course if there is no live input to the microswitch then there can be no output from it.
At a guess you appear to be describing a potterton suprima which has a history of sub standard 'printed circuit boards' caused by cracked soldering where terminal posts are attached. If you have poor connections then it may explain why you no voltage reading at the air pressure switch.