I'm definitely no expert but is there not an obvious source of damp e.g. a chimney or maybe an overflow outside which is continually dribbling (assuming this is an outside wall...)?
Try the Checkatrade website for recommendations for professionals in your area. They have to pay quit a few hundred to be in there and you can see peoples comments.
www.checkatrade.com/
Last edited by: smokie on Thu 22 Sep 16 at 23:16
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