I found this on another website which suggests the fan may be the issue
It's quite common for the fan to stop working on this model, was the boiler getting quite noisy ? vibrating ?
If the fan has been vibrating then it's quite possible that one of the wires going to the fan has broken, usualy at the spade connector, but its not always obvious, as the wire breaks inside its casing.
Or it could be the pressure switch, which operates the fan.
there are other reasons, but I think this is most likley to be your problem.
easy job for a gas engineer.