I just fixed it.

The problem:
It is six years old. Gradually, between firings, it took longer and
longer to light. This was because the pressure switch was more and more
reluctant to close (it would work immediately when the inner panel was
removed - evidently the air-flow is less restricted - the intake comes
straight from the room)

Everything looked OK. So I took the fan assembly out (3 screws + 2
screws on the collar) and removed the venturi (the top air connection
to the pressure switch - held in place with a 12mm nut). It had some
verdigris but otherwise looked perhaps a little dirty but it was not
blocked.

The fix:
I ran a piece of string to and fro vigorously through the venturi, put
it all back together and it now fires immediately everytime.

Cheers
Robin