I see your point @JohnD but I was talking about same boiler, it is clear a 1980's boiler replaced with new will jump 75% to 90% plus so real saving. But that 90% plus run flat out on/off or mark/space regulated, to run efficiently modulated is the real question. Same boil but different controls.
Maybe my English was not plain, remember the signs "Beware of the man eating Haggis"?
Maybe I should have said when it is condensing.
Do you have a plume at the terminal? If so you are not harvesting all the latent heat. The latent heat amounts to 0.93KW/cu m and the EU reckons you're lucky to get around 0.4kw of that if your return is 41 degrees.
Controls may modulate the boiler flow and of course return which if low enough provides the opportunity for condensation on the exchanger surface but this only happens if enough sensible heat has been removed first. e.g the return enters the boiler at 40 but one exchanger may have 2% of it's surface at that low temperature a better exchanger 5%.