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.