Not that I can see..

How exactly does this flue gas analyser determine the efficiency anyway ?

Surely the only way to measure efficiency properly would be to measure gas flow rate, (energy in) output and return flow temperatures and flow volume. (energy out)

If you know the temperature differential between flow and return, and the flow rate you can calculate the total amount of energy being put into the circulating water by the boiler heat exchanger, and the total amount of energy going into the boiler is indicated by the gas meter flow rate. This would give a very accurate determination.

I can't see how you could get an accurate determination simply by measuring the flue gas contents. An approximation based on a number of assumptions perhaps.