Keith L, Has the Flat got a Pitched or Flat Roof? If Pitched is it slate tiles? There maybe damage to the slate/s or one or several maybe missing causing water to come into the building. If it is a flat roof there maybe damage to the Felt roof or it has reached the end of its life, Also Check Weatherboards to see if they are damaged or rotten if they are replace them as water will be coming in from there. Also If it is a Sky Dish that was not sealed properly or a low grade Silicon was used remove all of it and redo the job with a good quality Silicon any builders merchants (MKM, Travis Perkins, Jewsons, etc.) will help you get a good quality Silicon.