I'd be looking at the chimney pointing, flashing and capping, and making sure that is all in good condition. Fix that and you ought to fix water ingress. A leaking roof or leaking or overflowing
can also cause this sort of issue.
Ground level issues are often caused by things like applying waterproof render or paint to walls which should breathe, thereby causing condensation problems, perished bricks, or obvious physical breaches of the original slate DPC. Welsh slate lasts for thousands of years and is almost always fine.