I live in a terraced house, built 1935, with a relatively new roof. As the picture shows, there is a bay with a gabled roof over beside another bay with flat felt roof. I have water damage to the part of the wall just under where the roof valley meets the flat roof, so I suspect water is coming in under the lead where it joins to the flat roof. I haven't been up there yet, not too keen TBH, but the lead looks reasonably fresh, so I think my guess is right.
How should a watertight roof joint/transition be constructed?
I would prefer to do it (or have it done) the right way, rather than just having some silicon slathered on, and if I pay somebody, I'd like to have some idea about how it should be done.