Damp/black inside bay window

1 Apr 2015
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United Kingdom

Apologies if I'm posting this in the wrong forum!

I've just been removing the window sill and wood cladding from an upstairs bay window. They were covered in black mould, as was the sealant filling the gaps between the sill and the windows.
Behind the sill (in the void between exterior which is some kind of soft stone? and interior which is plasterboard around the bay) I can see it's covered in what looks like dried up mould. It's dusty and black. Same between the window frames, which you can see in the photo below.

Is this normal, residual signs of a condensation problem, or something more serious that needs to be treated?

The house is approx. 100 years old, no idea when the uPVC windows were installed but I would guess a few decades ago...
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