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oiver - IMO the calibre book editor Links Report is 'better' than Sigil's.
The calibre book editor is implemented as 'stand alone' program so it can be used without using the GUI library manager (although it must be installed). On Windows its at C:\Program Files(or Program Files (X86) for 32bit version)\Calibre2\ebook-edit.exe - not sure where it lives on Linux or Mac. There's also a 'portable' version of calibre for Windows.
Pity the calibre editor and viewer aren't published as a separate package
BR