It's on the entrance of the lean to, the outside wall has been rendered last year and the lead on the lean to roof seems fine - I'll take pictures of the outside as soon as I can.
There is an old chimney breast in this wall but the chimney was removed before I bought the house, could this be an issue?
Yes a chimney probably has a lot of salts that absorb water from the air, it's probably soaked through to the surface now.
Unfortunately unless some suggestions come from when you show us the other side, I think you will have to chop off the plaster and redo it. You can get special solution to neutralise the salts before you ree plaster it again.
We had the same problem when they knocked off our chimney, that's how I know. Yours does look exactly like the shadow of a flue.
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