I'll speak to Building Control Tuesday.
Good point, Lobster re potential rotting wood. I could go out through
the wall and add a rt angle plus vertical extension flue to avoid the
eaves.
I'll consider going straight up through the ceiling and roof if I can
do it without trimming joists/rafters.
Owain, I could hire a flue gas analyser. Would it be for 'information
only', or would I be able to make adjustments in the boiler to correct
the emissions -eg adjust air/gas ratio for better combustion.

Thanks to all,

Tony