Thanks for the advice, folks I've tried to attach the relevant files, but the group is too large (7.5MB compressed with 7z) to do so.
Regarding texture view being misleading, I fully agree and have on many occasions had cause to regret it! In this case I am a little stumped though: I can't see the error, if I have made one! I have checked and double-checked the assignments and can't see the error with the invisible end face (though I did find a couple of others which fixed some issues
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I have checked the normals: as far as I can tell they area all fine. I've only one UV map and part of my double-checking was to set it explicitly. I did catch a couple of 'left at "Generate" rather than "UV" errors that way, but sadly that didn't fix the main issue. If I could trouble you for a look at the file?
Edit: I've attached a pic of the current state
Ruth