Glow.worm Ultimate 120ff somtimes not firing up?

My Ultimate 120ff will sometimes not fire up. it hapens sometimes once a day sometimes it will run perfectly for 2 days. When it will not fire up, it is after it has not run for several hours and is stone cold. When it failes to fire I have checked the 4 neons and none of them are lit. At this stage if I turn the power off and on to the boiler several times it will fire up and run with no further problems. I have considered changing the PCB but do not want to wast £100 if that is not the fault. I am now thinking more of the air presure swich as the fault only occours when the boiler has been off for several hours and is cold.