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added a comment - keith has advised that the damp has got in around the window, and panel joints just about each corner. The small section or wallboard above the cill is also rotten.
Lower window will need to be removed and possibly top one as well. Woodwork needs to be assessed and repaired. New cill needs making and replacing
side part to corner unit would have to be waxed
Need cill board 1600x150 Bigger the better
Small piece of wallboard creamy yellow
timber for framework 30x30x6m
Silicone