He also said that modern boilers used insulation to stop wasting heat by
transfer into the surrounding space and that they do achieve lower flue
temperatures which means they are extracting more heat from the gas.

Indeed, although it is the much larger, more efficient heat exchanger which
does this, rather than insulation.

A well designed condensing boiler installation might have a flue temperature
of 50C, compared to an old boiler with 300C. The flue temperature being
considerably lower than 100C is the cause of all the pluming. Although
people worry about it, thinking it is a health hazard, the emissions in that
plume are much better than those in the invisible 300C one. But you can't
see the 300C version, so no-one cares.

Christian.