It could be the fan proving switch then. This located at the back of the boiler and is accessible from the top right hand side. The switch is factory sealed so you can't really repair it but from experience, I have found that removing it and giving it a few gentle knocks with the back of a screwdriver, it may start working again. If this solves the problem, be prepared to replace the part soon as it will probably happen again.
John
November 2006