Gas engineers are required to be able to
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.