Had a Worcester 19/24 CBi which wouldn't fire up today. The fan would run
but no ignition. I pulled off the tubes from the pressure switch and
blew and sucked into them and the pressure switch clicked. After
that (after reconnecting the tubes) the boiler fired up OK.

The fan ran smoothly and quietly so I'm guessing the pressure switch is
getting a bit dodgy, but before replacing that I thought I'd ask
around in case there's some subtle problem with the fan on these
machines. Geoff? (And if it is the switch do you do those too?)