Sime Super 90 MkII fails to ignite 'sometimes'?

My boiler 'sometimes' fails to ignite. I have my boiler on timer early morning / late evening and notice sometimes during the Central Heating (CH) cycling on/off, that it sometimes fails to ignite. Once it fails to ignite once, it does not reignite when it is next fired up as it continually stays on (by that I mean it makes the same sort of soft humming sound it would make when the CH is on). Boiler is about 10yrs old and has been regularly serviced. The ignition failure sometimes occurs with Hot Water demand as well but less frequent. I simply turn the boiler off and on again to remedy this but I would say the same problem reoccurs 2 -3 times per week (only in winter period when CH is required).