Potterton Netaheat 10/16 Electronic sudden failure to light up?

Can anyone shed any light on this sudden failure, please.
Boiler fan runs but gas fails to start.
Fan working OK - it was repaced about 12 months ago. Good air flow out of exhaust flue vent, no untoward noises!
Tried spare pcb - no difference, no start up.
Read other responses on here and found that blowing into one of the pipes to the air valve starts boiler OK. Blowing into the other pipe has no effect. Is this correct? What is the function of these two pipes - apart from the obvious answer i.e. detecting pressures?
Tried reassembling cover carefully to get good seal; no different.
Can anyone advise, please?
Thanks in anticipation,
Ian