Gas engineers are required to be able to see the flue to inspect it if your boiler is on an outside wall, it's unlikely you have a hidden flue if your gas engineer can see all of the flue, then you don't need to take any further action. The efficiency of a boiler is measured by its afue (annual fuel utilization efficiency) rating this rating is a measurement of how much heat a boiler produces vs how much fuel it consumes depending.