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I think I have tracked down the problem - I am fairly sure it was one of the Firefox add-ons which I was using - probably Hola, though I am not sure as I tried disabling two or three of them (the others were ad-blockers) at the same time. Anyway, after re-starting the file now appears as an epub.
Nevertheless, it is a curious side-effect!
Thank you all for the suggestions.
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