DiapDealer created an unofficial debian package of Sigil 0.7.3, here:
https://www.mobileread.com/forums/sho...d.php?t=211754
Just download the correct version (32-bit vs 64-bit) and double-click the .deb you downloaded. It will automatically open in the Software Center and comes with a nice shiny install button!
This is the copy I am running, and it works great on Ubuntu 12.04 LTS -- the repository suggested above isn't available for 12.04 only 13.10.