Worcester Boiler - Combustion fan keeps running, Why?

Good Morning,

I have a Worcester Boiler, model - Danesmoor 15/19 (HPJ.19000.) and it's working OK apart from the fact that the combustion air fan does not go off. As the pump is coupled directly to the fan I'm nervous that this will wear out quickly, therefore, I'm only switching it on when needed. I believe the Pump over run relay logic board may be at fault? But I'm nt sure as I don't have any drawings. Any help or advice would be gratefull.

Kind regards
Peter Davies