Thanks Andy and Ian Stirling for the response. Andy, you're correct,
there are no roof options. The flue is coaxial and comes out of a
Chaffoteaux & Maury boiler. I'll be able to get hold of more details
later today.

Warm water sometimes comes on. I can't figure out a causal explanation
for this, but suspect (speaking as a complete ignoramus) that it has
something to do with airflows on the street side. I guess that we're
somewhere on the border between it working and it not working. That's
why one of the things I was interested in exploring was the possiblity
of using a different vent. The one the workmen slapped on before taking
down the scaffolding (4th floor flat...) is a tight grille, which I
guess has roughly the effect of sticking a pigeon in the tube. One
obvious solution would be to tear off the vent, but I suspect this
would not go down well with the gas company, who are due shortly to
give the installation a 4-yearly safety check.

To the rest of you: Yes, people here often do take receipt of the
subsidies and then undo all the (aesthetic) improvements the council
told them to make, putting the airco back on the balcony etc etc etc.
Unfortunately the incredibly slow bureaucracy here means that subsidies
take up to a year to be adjudicated, by which time the nifty Italian
woman in the front room will probably have drawn her own conclusions.
Mexico is unlikely to be better than Spain in this respect, and I
believe it rains in the UK.