Chris Plastow
Further investigation may be required but..... If the chimney was ever used for a coal fire then it contains ammonium sulphate. One will get condensation within the chimney flue , particularly when it rains. This will cause a staining to the decoration. Waterproof render will stop the damp( stain) but the flue needs ventilation too or it will just get worse. Lastly , the gas flue could be leaking at the top .
