Why does my Aplha 280E Boiler fail to light now & again?

My Alpha 280E boiler has the following problem -9 times out of 10 it is OK. Sometimes, you switch on the combi as normal [hot water say] -pump comes on, all sounds good but no ignition! A plumber told me it could be the DHW flow valve diaphragm -on inspection both plunger valves [inc. primary flow] work OK [he also though it could be the whole assembly [both valves were working fine when he looked at it so was difficult to diagnose]. What else could it be -a sensor?