Problem with Vokera 80SP combi boiler?

My Vokera Excell 80SP has developed a fault in which I have no hot water or central heating. The pilot light is lit and the fan runs. When the hot water or central heating is turned on the 3 port diverter valve and the primary flow switch activate, the pump and fan both run but the boiler fails to fire up. I have received a reply to suggest the primary flow switch diaphragm may have failed but this doesn't seem to be the case as it appears to be operating correctly. Is there a simple/cheap fix? I am very grateful for all the replies.