Could be between the frame and the render, with the render holding the water against the brickwork and not permitting it to dry out.
What is the guttering like? The top looks greenish - if water is coming down, it could be running down into the gap between frame and wall.
What is underneath the decking - is there any drainage?
How are the walls built?
@NotThamesWater started
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Could be between the frame and the render, with the render holding the water against the brickwork and not permitting it to dry out.
What is the guttering like? The top looks greenish - if water is coming down, it could be running down into the gap between frame and wall.
What is underneath the decking - is there any drainage?
How are the walls built?