potterton prima f?

Hi,have a problem where boiler fires up,everything ok then flame on burner starts lifting away,then a click & flame goes out,the rest of the system still going,another click & burner fires back up.This happens randomly.Engineer says flue is corroded & blocked (without looking down it!) & needs replacing.Ihad a look,there was a bird in exhaust! removed bird, fault still there.I can see right down flue & its only very end of exhaust that has dropped slightly out of terminal.Engineer struggled to remove flue,ending up hacksawing between boiler & wall!!! My opinion was boiler needed to come off wall.Being in the motor trade I understand flow of gasses & feel after a big bill fault will not be cured.Any advice would be great.