My 350 combi is not lighting at all, the same usual sequence of clicks and ticks, the fan starts and runs but you can not hear the usual clicking from the ignition/spark to light the burner. It just will not light and the fan just keeps running all the time, no hot water, no central heating, plumber blow away from air pressure switch reconnected with hot tap running bolier lit up turn tap off shuts down won't start again any help direction please.