@Kovid - not sure we're on the same page.
My bottom line is that I don't want the spell checker to check the spelling in Comments, that's because they are full of my own ad-hoc abbreviations. I do not want to correct them, nor create a user dictionary for them; in fact I don't want to see them. In one instance yesterday the calibre spell checker reported 87 errors - of which 83 were in the dc:description element. In Sigil all I saw were the 4 genuine errors, I assume that's because it doesn't check the content.opf file.
My problem could be overcome by providing an option to exclude files from being spell checked.
If that's not possible I'll continue to use Sigil to spell check and do other edits (which are few and far between), I'll use Calibre to do link checking though, for some reason I never got my head around the Sigil equivalent
BR