We have a detached house built in 2005. The brace wall in on the outside of the house. On a wall upstairs where the chimney would be has become boast in 2 spots each about 10cm in diameter. Someone has diagnosed that we have dirty wall ties, someone else has said that we need to put a cap on the chimney and put on fires to dry it out. Which is it most likely to be and how much is it going to cost us to put right?