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