think ive found the solution, i have two windows with this problem, one with an opening.
someone on a double glazing forum reckoned the windows were not sealed properly with silicone at the bottom of the frame:, heres what they said:
I have seen the dampness at the bottom of the plaster line indoors in the bottom corners MANY times, and the reason for it is usually that the bottom of the window frame itself at the extreme edges of the frame were not sealed with silicone to the cill, front to back, when installed.
Therefore any ingress of water past the sealed unit gaskets - worse with lead lights - (which is normal - that is why windows are drained) travels PAST along the bottom corners of the frame (instead of being stopped by a silicone line) going away through either face drainage, or bottom hidden drainage.
Make sure that some uneducated person has NOT run a silicone line outdoors between the window frame and cill, there SHOULD be about a 3mm gap between the two, with concealed drainage. Personally I far prefer face drain...
The cure (maybe a bit of a retrospective cure - but it will work if done properly and VERY carefully), is to remove the sealed unit, drill about an 8mm maybe even a 10 mm hole right through the window frame (so you can ger the nozzle right down) through all chambers at each bottom frame corner, until the drill bit has gone down to on TOP of the cill - NO FURTHER. Take great care about how far you drill!!!
THEN, pump a GOOD portion of silicone in until it comes out of the hole you drilled, effectively and retrospectively doing the job right in the first place.
Drainage water, I reckon, is passing beyond your frame horizontally in the corners and making your plaster wet, because the frame was not sealed to the cill at the extreme bottom corners at time of fitting.
Seen it loads of times, although I could be wrong, I don't think so.