Perhaps I'm missing something but they talk about using wasted
heat to generate electricity. But the best condensing boilers
are already running at 91% efficiency, so there's not much to
reclaim, certainly not 25% ????

It's more like the wasted heat from generating electricity is used to heat
the water. This means that even, say, a 50% efficiency generating
electricity doesn't matter, as (a) gas is a quarter of the price of
electricity and (b) the waste from this heats up your water cylinder anyway
(usually supplemented by a standard burner as well). It's a win-win
situation.

Unfortunately, I can't install it now myself, as I never install first
generation products, and it seems to be in zeroth generation at the moment.
I've plumped for the Bosch Greenstar 28HE instead.

Christian.