Problem with coving and window frame

[Aurelio]

I’ve noticed a water mark on the coving above the window frame in our bedroom, the frame(UPVC, was installed before we moved here) seems to be coming away from the wall slightly.

There are water marks behind the frame. Ive checked the roof and no noticeable signs of leaks. The exterior seems to be ok, it was repainted in lime last year. Any advice on how to deal with this please?

 

[Cubist]

The plastic cover over the UPVC window frame is merely window dressing and serves no other functional purpose. Driven rain may be penetrating indoors via poor/non-existent caulking around the window frame proper. You need yo examine this and repair the caulking as needed.

 

[paulc]

Water has an annoying ability to travel from the point of entry to an area on the inside. Whilst it could be poor sealing of the frame outside, the top should be reasonably protected from rain by the overhang of the roof. With that in mind, I'd be looking for slipped tiles/slates, particularly on the bottom row.
With the trim coming away from around the window, I'd be tempted to pull it off. Check that there are no gaps between frame & brickwork (filling if any are found) and patch up the plaster underneath. With a solid base, refit the trims either reusing the original uPVC, or with a suitable timber profile.