Originally Posted by MrB
Please do not interpret three posts in relatively rapid succession as nagging or desperation but I seem to be doing something wrong with my thin spaces.
I have used both &# 8239; (without the space) and   at various points in my French sentence. As expected, their effect is identical. However, their effect is also unexpected!
In Sigil the thin spaces appear as a very bold, fat letter I.
In ADE the thin spaces appear as question marks.
What basic, foolish error have I made now?
The only error is that you did not read my post just ahead of the one you posted. The problem is that the reading application, i.e. Sigil and ADE does not have a nonbreaking thinspace or perhaps even a thinspace in the character fonts it is using. Sigil has some other character for that number and ADE has nothing so it presents a ? character. You need to use a small or else embed a font character set that has what you want. Personally I would go with shrinking a space.
Another possible error is that the file is not unicode or is not specified as unicode.