Leak?

Not long after I moved into my flat about a year ago, a wet patch started to appear at the top of one of my walls.

My upstairs neighbor has been looking a bit but hasn't found anything. It's where his kitchen is so he's been looking under the floor with a little camera rather than taking any units out.

My flat is a garden flat, so a bit low down, but it is a middle terrace, so the wall is shared with nextdoor. I don't think it could be a leak from the outside, as it's about 2m from the external wall.

I have been heating and dehumidifying the flat (55-65 RH and 19-21 degC always). Is there any danger is could be a condensation problem? I would have thought it would form on an external wall, or lower down, or in a less well ventilated area (I have built-in wardrobes).

The building is a late victorian town house, so construction is brick and that 'orrible black plaster stuff.

I'm basically after some reassurance that it couldn't be anything else before I insist that my upstairs neighbor gets a plumber in to investigate (as he's been dragging his feet for the best part of a year about it; getting his dad in every couple of months to poke around with him without doing any real investigative work). The building's water supply comes from the back corner of the room and his waste pipe also exits the building at this point, so basically all water coming and and going out of his flat goes over the area in question.