There is currently no repo/ppa version of Sigil 0.7.x available. Working versions anyway. That's because the Qt5 dependency isn't able to be resolved using standard repos and ppas. If you want to run version 0.7.0 and higher, you will need to install Qt5 manually and build Sigil from source using the instructions provided on
Sigil's code.google home.
Either that, or try the unofficial binary debian packages I built that people are reporting good luck with so far.
https://www.mobileread.com/forums/sho...d.php?t=211754