Damp raised alcove: condensation below the floor in plaster?

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16 Nov 2013
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Kent
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We've bought a 1930 house at the end of July, with two surveys not finding any damp problems.
After living a couple of weeks in the house (and after decorating with a vinyl trade emulsion), a damp path appeared in an alcove of our dinning room, between a vented chimney and the outside wall. First it was damp about 30cms above the floor on the party wall only, then it spread and now the whole side of the chimney and the party wall are damp, from the floor to a height of 55cm.
The alcove is raised, such that its floor is same height as the top of the skirting board. I took out the floorboards of the alcove, and saw that the walls there are plaster to normal skirting board height, but that is below the alcove's floor!
So I am wondering whether condensation in the plaster below the floor could cause a damp alcove up to 55cm height?
I hacked that plaster off, and now see that the pointing on the inside of the outside cavity wall is completely rotten- it's like topsoil in our garden and even looks soaking wet. So my second question is, what to do about that? Can I just repoint it? I had repointed the outside of that wall a couple of weeks ago, but now wonder whether the cavity might be full of the same sludge as is now between the bricks inside. If so, how would I clean the cavity? From an airbrick? There's one 80cms away from the corner of the alcove.

Many thanks!
 
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