This is not an exhaustive or a complete subject. It is likely to change over time, with edits, additions and deletions, and is simply a collection of my thoughts based on many years of experience. You should follow your instincts, not rely at all on any of this and you must always seek your own professional advice from a skilled, experienced and fully qualified building surveyor.
A few years ago, I recorded this short video which describes and illustrates the often misdiagnosed consequences of sulphate damage.
It is worth taking another look, before assuming that the chimney is leaking.
Of course, it might be a leak but before you jump to the wrong conclusion take a look at this video
Disclaimer: Anything posted in this Blog is for general information only and it is not in any way intended to provide any advice, legal or otherwise, on any general or specific matter that you can rely on. You should always seek your own legal advice.