Damp in corner of ceiling

[Olga]

I live in Victorian mid terrace house. I have a damp stain in the corner of my ceiling (outward facing wall adjacent to neighbouring property). I have had the slates on roof replaced to ensure that they overhang the guttering. The neighbouring join tiles have also been replaced.

There is no other damp apart from in this corner and it has not spread down the wall and appears to be spreading very slowly inwards.

Does anyone have an idea of what could be causing this and how to treat it?