Thankyou for replying to me. The stack was taken down immediately we moved into the house as there was lots of water leaking down the wall. Although the leak appeared to have been solved the area remained damp. We were told to have the ceiling and wall replastered as the plaster would be holding the moisture in. We also had a new roof installed around the same time and thought that would solve the issue but it hasn’t. I am attaching a photo of the outside of the stack. The render has also been redone.