Hello Simon,
Following points will be helpful to you.
I am happy with the sidebar width but want to change all pages main section to be about 15% wider if possible?
Contango is based on Skeleton Framework and it is not easy to make it wider.
Is it also possible to make the boxes in the main and siderbar actually touch and not have the gap?
Every theme has its own design, Choosing Wider layout may have little change in Contango appearance to fit your taste Example: http://themes.designorbital.com/contango/blog/
Try Wide Layout in Appearance > Contango Options > Layout Settings
Most Important
DesignOrbital themes are very easy to customize. You may attain your required changes like Sidebar Without Gap by playing in the relevant sections of style.css
Thanks!