I believe this the new(ish) floor standing boiler?
You can't replace the fan pcb, it's part of the fan itself.
Replacement of the fan is reasonably logical but if you don't have a gas analyser to reset the burning configuration on completion, may I suggest you don't go ahead but get a Gas safe engineer to carry out the work? If you don't set the CO2 ratios, you could be sooted up within a week if you don't know what you're up to.
A more pressing issue is the ingress of water through the flue, if this isn't sorted, it will only re-occur. Was this boiler even fitted by a competent person? I have my doubts.
I'll refrain from commenting further.
Carefree
July 2010