I found out this week that condenser gas boilers only work at high efficiency if the water coming back is at a low enough temperature. The outgoing temp is often set at 70 + degrees. This means it's too hot for the energy saving condenser action to take place.
Read up on it but it seems you need to drop it to 60 ( or even 50 in a well insulated new build). This will generate enough heat to heat and support condenser function.
It seems that boilers are often set to these high temps as they were needed with older systems, not needed now and prevent you achieving the efficiency you should.