Problem with Britony Combi SE 100?

Boiler lights, stays on for about 1-2 mins but then goes off and the 40,60,80 lights flash. Fan seems to make a noise and we found that seal between fan assembly unit and flue had perished. found a make shift seal to replace temporarily but still same problem. We are unsure if the seal is the only problem or if the fan assembly needs replacing or air pressure switch??