Thanks for the thoughts, all. I think this is one of those "make a plan, execute it one step at a time" situations.
So I'll make a few calls today and see what I can set up.
I don't really know the cause, although I have a suspicion it might be something to do with the chimeny, which is on that wall immediately above it. It;s on an end wall, so no guttering above it.
I'll probably try to find a "chartered building surveyor", as helpfully labelled upthread, because I want a proper diagnosis and solution.
I guess it will be somewhere between "there's a leak in the chimney, fix that and wait for it to dry" and "that whole end wall needs rebuilding".
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