Chris Plastow
The staining on the chimney breast is unlikely to be from a leak to the chimney . When fossil fuel is burned in oxygen , a salt called ammonium sulphate is given off , this is absorbed by the cement within the chimney flue . It is a salt , which means it it will absorb moisture from the atmosphere and cause dampness in the plaster / render . It would be fine if a fire was lit sometimes as this would keep the chimney dry . I suspect your chimney lacks ventilation , which causes condensation within the flue to increase , this is called interstitial condensation . Ventilate the chimney at top and bottom to prevent this . Unfortunately , the staining you observe is amonium sulphate and will have to be repaired by removing the render to the brick work and replaced with a waterproof render , waterproof renders stop salt migration . Your builder won't agree with me ! Good luck
