Worcester combi boiler not always firing

We've got a Worcester 350 Combi (natural gas) boiler, which I think is about 10-11 years old. The problem is that intermittently, it fails to fire.
When the hot tap is turned on or the heating is switched on, there are a few (relay-like) clicks, the fan starts up and then the spark starts. After a few seconds it gives up and there is another relay click. The fan stays on, but there is no flame. This only happens about 1 time every 4 or so; so it's just a minor irritation at the moment.
I'd be most grateful for any ideas on what the problem is, and if it's likely to be a DIY job. Even if it's not, it would be nice to know what's wrong if I have to call a heating technician, and how much it might cost.
TIA.
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Stripped it down and cleaned it (not that it was very dirty) and measured the gap from the spark electrode to the surface of the burners and set it the mid point of the range in my handbook. Its worked fine ever since.
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Thanks, I'll have a look in the handbook. Any obvious gotchas with stripping it down?
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Not as I recall. I just remember it me thinking that it *had* to be harder than it actually was. I guess you should be careful how you re tighten the plate back on to the front.
I guess you should remember that the law say's you have to be *competent* to fiddle with things gas.
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Thanks for the tips.
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