Water ingress under window - need advice please

Sambi

Images are at imgur.com/a/ZMFMoP1 (sorry I can't make that a link as a new user).

I have a damp issue which I think is caused by water seeping into the wall through cracks in the window frame. When I moved in a month ago it was all dry, the dampness only appeared after the heavy rains recently. I am slightly puzzled why the dampness travelled across onto the green wall, I'm guessing it must have pooled at the bottom of the wall and soaked up? The green wall used to be a chimney stack, but I don't think there's water coming in from above, or the pattern would be different.

I was thinking of re-sealing the bottom part of the frame with caulk, then drying out the wall to see if that fixes the problem. Longer term I might have to have the window repaired/replaced and the wall replastered, but I'm looking for a stop-gap solution for the moment.

Is this a good idea? Do you have any extra suggestions? Thank you!

Jonesya

If it was me I'd check around outside first to ensure there's no issues with water leaking from gutters or downpipes (that might be making it worse), scrape down, clean and re-seal the window frame, using an outdoor caulk, I'd also give it a coat of paint.

For the damp on that green wall, if that is the fireplace and chimney behind it, then there should be a void, so if that damp is coming from outside then is it crossing both the exterior wall and the void behind the fireplace? I'd be wondering if you've have water coming down the chimney and either running down or pooling in that corner. Could you unscrew the vent cover and see inside, check for damp?