Ariston Eurocombi boiler fails to fire?

Hi
I have an Ariston Euro Combi boiler. For some time the burners have been slow to fire up on demand (on both hot water & central heating). The fan starts but the burners do not fire for a time and this time lag has been growing for the last few weeks.
Recently, the burners stoped firing all together. I removed the front cover and then the large cover to the front of the boiler (but not the smaller cover to the burners themselves). When I did this and turned on the hot tap the burners fired. However, when I replaced the cover, again, the burners failed to fire. Eventually, I found that with the cover in place, if I remove the panel on the top left of the boiler which houses the pressure switch the burners would fire but, with the panel in place, they would not. In other words, it seems that if I allow air in to the unit it fires but, if I replace the panel, the burners do not fire.
Can you help please and tell me what the problem actually is.