Faulty Myson Apollo Fanfair boiler?

It appears that the fan on the boiler keeps operating every 3 or 4 minutes for approx 30 seconds and then shuts off.This happens when the hot water or radiators are both switched off at the honeywell timer.
Therefore when the fan starts it does not fire up the burners - unless the pilot light is a real sooty flame and the fan is just sucking away the waste gases ?
Anybody offer any help ?

Gazla