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Thanks everyone. I don't think it's condensation, Old_Geezer, and I don't have a basement . What's worrying me is that it might be rising damp as a couple of you have suggested. I've looked at the outer wall, johnk, and I do have plants growing up that might be bridging the damp proof course. I think the central heating is OK. One thing I have noticed this evening is that the front garden is very wet, even after two days of sunshine, and I wonder if there might be a leaking water pipe. I'll get Thames Water to check.