We have a lot of experience with salt contaminated chimneys, and I can offer you the following insights:

Due to the very nature of how chimneys are constructed (whether in use or redundant) they are probably one of the more challenging damp problems known to remedy and one to be most known for recurrent problems. This is still found to be the case even when flues have been totally removed and brick in-filled but then replastered (often incorrectly with gypsum based backing coats).

Moisture that has come from a chimney certainly will and always will contain salts. (Whether that is by condensation or ingress, any moisture - however it enters a flue -causes problems). The moisture combines with sulphates deposited inside the flue as a result of combustion to form weak acids. These acids attack the lime in the parging and the mortar joints, as well as the brickwork itself. They also form hygroscopic salts (salts which attract yet more moisture). Therefore even when a flue has been remedied of damp or ingress (or, as previously stated, completely removed) the
visible damp spots on internal walls will continue to reappear in damp and humid conditions unless the hygroscopic salt contamination is treated accordingly.

Effectively the best known safeguard against further salt contamination and decorative spoiling over a salt contaminated chimney breast is complete removal of plaster/render back to brick substrate and full 3:1 waterproofed render and tanking with Vandex BB75. This process installs sulphate resistance whilst also maintaining vapour permeability, which is extremely important. Additionally, redundant stacks need to be capped and ventilated according to Building Regulation ‘Passive Stack Ventilation and Ingress Control’.

if the old bricks in the loft from the chimney are in contact with the felt they will draw in water from the felt if not repointing and a weather bond waterproof coat painted on the out side brickwork as they are Victorian bricks they may be starting to fail and draw in water