Mr Mogey, good evening.
See it is your first post in here? if no one else does Welcome.
The marks appear to be a dampness issue?
Could be two things, one is Condensation occurring internally, or could be penetrating Damp where water is driving through the external wall, problem can be a break down of the DPM around the lintel.
A couple of questions that will assist in possible answers.
1/. What state is the Lintel in externally? is it cracked?
2/. If there is external render or pebble dash is there any cracking in it around / above the Window?
3/. What age is the property?
4/ What state is the Mastic around the window in?
5/. Any possibility of an external photo of the area above the window?
Sorry to ask all these questions, it is simply what i would do if I an working in a property to try to identify source / cause / remedy.
Ken