alpha HE CB25 2 amp fuse blows?

hi
I have an alpha HE CB25 boiler I was in the shower and the boiler stopped working, it had blown the 2 amp fuse inside the boiler.
I replaced the fuse turned it on and it blew again.
When I put a new fuse in, the fan comes on and I hear a few ticks, then just before it fires up, it blows.
I was told it could be the fan so i replaced it, but the same thing happens again. I have noticed that when I replace the fuse, and turn on the boiler, there is a large spark from the top of the motor of the fan (but not coming from the mains wires).
2 years ago we had a similar problem with the 2 amp fuse blowing, and it turned out to be a problem sensor.
Can anyone suggest any advice?
Thanks

Steve L
March 2011