I think you may have answered your own question here. I have no
idea why it would stick 50cm out from the wall, if a normal coaxial
flue, and though I haven't looked at the pdf file, I suspect it will show
a streamlined looking cowl poking no more than 10cm out of the wall, if
that.
It may be possible to mount it completely flush with the wall, especially if
you
flare the opening in the masonry slightly. You need to examine the regs for
the positioning of flues, there may well be one in the pdf files on the
chaffoteaux
website.

Andy, thanks very much for your help. I looked on the previous page of
the installation booklet and they seem to want the exterior flue tube
to protrude slightly from the outside wall and then the interior flue
tube to protrude from the exterior. Your suggestion of recessing the
whole caboodle makes sense to me (speaking as a fairly dazed novice)
and then perhaps banging a highly permeable vent on the outside to
satisfy the grille fetishists at city hall. I've taken the liberty of
drawing a picture (sorry, it's a stoopid BMP) of what I think it would
look like: http://oreneta.com/baldie/flue.bmp

The legal angle is being pursued, although our man seems to have fled
his wife and taken refuge somewhere out in wine country. I believe this
is also a Spanish Practice.