Thanks for all the comments - as always the subject of condensing boilers
seems to stir a reply :-)

I appreciate the "condensing boiler is always more efficient" argument but
my friend is running a small hotel and been quoted (rough figures)
£800 for a conventional boiler to do the job and £1600 for a condensing
boiler. So if the efficiency advantage isn't fairly large the payback time
is
going to be long. It seems that there really is a choice to be made here.

As to my "silly" idea of putting their pool heat exchanger in the return
loop
I'm still not sure how silly it really is.

The point made about control rather than efficiency is a good one but
control could be added with couple of valves and a thermostat.
I can also see that there is a problem with a modulating boiler thinking
that
the house was still cold because the pool exchanger was still lowering the
temperature and I'm still thinking about this one.
Given the house has termo valves on all the radiators and so does the pool -
how
does this change things?

But after saying this I now think that the general vote of "it isn't worth
the gain in efficiency for the complexity increase" - but I thought it was
a nice idea.

mikej