What's the likely cause of this damp?

[gwernybwch]

This is a photo of damp in a living room in a Victorian house. The wall is an external wall.
The house had the 1m hack-off, an injection DPC and replastered about 19 years ago.
The horizontal line of the damp would be around where the old plaster meets the new plaster.
I can't see anywhere obvious where there would be water ingress from where the window meets the wall. Similarly I can't see anywhere noticeable where the external rendering is blown.

Possibility that there is a hairline crack on the outside that is allowing the water in and it works its way down before hitting the new plaster and then goes horizontally?
It wouldn't be the case that it comes in by the window working horizontally and goes up vertically?
The room above is the bathroom, so possibly there is a leak in the pipework under the tiled floor.

Thanks in advance for any replies.

 

[helvellyn950m]

Hard to say without seeing in person, but it's generally accepted that gravity prevents "rising damp" from going any higher than 1m.

 

[Hugh Jaleak]

Any downpipes the other side of the wall?

 

[gwernybwch]

Thanks for the replies.

Hard to say without seeing in person, but it's generally accepted that gravity prevents "rising damp" from going any higher than 1m.

Do you think that it is possible for damp to enter (via a break in the seal of the window against the wall) and then work across horizontally and then vertically?

Any downpipes the other side of the wall?

That's a good point, outside there is a drain pipe and a soil stack that would run close to the vertical damp line (although I wouldn't say that either pipe is adjacent to the damp line). I've not seen there being any problems with the guttering or the drain pipe.