In message , Lobster
writes
Also; they wanted to do the job with a forward-tilting roof and
guttering installed; but because this is a period property I want it
doing as is, ie matching the other (original) dormers; that is, built
with no gutters and the roof tilting rearwards towards the tiles.

I really know nothing about these things, but we have a much larger
back-tilting dormer roof.
We consulted a surveyor, who advised that changing the tilt would
involve quite a bit of additional work at the main roof interface and
advised against it.
We had occasional water ingress from the "gutter" along the back of the
roof and he agreed with me that the way to deal with that was to take
the dormer roof covering much further back under the main roof and form
an upstand as a sort of dam. We presumed that in storm conditions waves
sometimes formed in the rain on the dormer and had been blowing up and
under the main roof.

Coincidentally, today as we drove in my wife remarked that the
re-roofing of the dormer all those years ago had worked well.
--
Bill