looks like the exterior of the window is not sealed properly to me.
How long ago were the internal walls plastered?
That definitely seems like penetrating water.
Can you get a shot/close up of the exterior and post up? Has this just happened or has it been like it since the window was replaced?
Hi, property was built in 1970 & the dampness is on 1st floor. The fitter is going to have a look tomorrow ( who is also my neighbour ).