The service engineer said the boiler (Potterton "Profile Prima")
as in good nick, which surprised me.
But he mentioned that there are now "condensing boilers"
which save a bit of heat, as they extract heat from the fumes
which are expelled outside the house in older models.

Generally speaking, it is neither economic nor environmentally friendly to
replace working models, unless they are particularly inefficient. The costs
(money and energy) of building a boiler are quite high and will take a long
time to offset against decreased fuel usage.

When the boiler does need replacing, you should replace with a condensing
model. Generally speaking, you are required to do so, but there are
exceptions, which you should not be tempted to use unless you really have
to.

Christian.