Thanks theducks and KevinH.
Quote:
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Earlier versions of Sigil did not properly enforce this url spec.
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That surely explains why I didn't pick this problem up when I wrote the first version in 2020/21. The LTS version of Linux that I was using in those days had a version of Sigil which was very out of date v0.9 ish.
I've bitten the bullet and renamed the image files without the + sign with corresponding edits in my image tracking system and now all is good.