Advice for single skin brick wall that adjoins stone wall?

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False widow?

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Is this a layer of dark hydraulic lime mortar directly on the stone or just cement?

This is a UK and ROI specific subreddit, r/DIY has a lot of US Specific content, things like products to use, where to get them, laws and codes that don't apply to the UK. r/DIYUK is for UK and ROI specific DIY help, questions and projects.