Jesse Clarke
Hi Jenny, This is an issue for a roofer. The problem is likely the pointing / flashing / cowl on the chimney but it could be due to broken slates next to the chimney. If it has been fully capped of then it could be condensation. Our details are on the internet at Max Roofings website. I don't know where you are but we are based in the west yorkshire area and cover Leeds, Bradford, Wakefield, Huddersfield, Halifax, Burnley, Blackburn and all surrounding areas. If you think we can help then please feel free to look us up and give me a call. Jesse.