Check the thickness of the walls to see if there is a cavity, if not get it externally insulated and rendered in something like K1. Also does the extension have a flat or pitched roof? Does it have a decent overhang? Water may be getting in from above. If the outer skin is breeze block, I'd have it rendered.
It has a slopping roof with a good 11 inch overhang slopping to the front and guttering along the front. and about one inch overhang on the side which is cracking. Will check when it rains to see if water is running off the side of the roof and down this wall, just occurred to me maybe this is happening. Will also check in roof space to see if damp is getting in.
The walls are a good 11 inches thick so assume they do have a cavity.
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