Chris Plastow
It might not be leaking at all! There is a condition called ammonium sulphate contamination , if the gas flue is in an old chimney , condensation will occur in the old chimney flue and cause brown staining or even wet patches on the chimney breast in random areas. If this is the case, ventilation to the old chimney is essential , the areas of dampness will reduce but in most cases replays teeing to those damp areas will be required , using a waterproof render . This is a little known condition and is the cause of great frustration as repeated attempts to 'stop the leak' fail!
