Footer width, and widget area interference

I’m using a child theme on the site rusper-pc.org.uk and I’m trying to set the footer width to 80% to match what I have set for the Header and top navbar but can’t seem to find the right entry to edit – any ideas?

Also, on pages where I am using the Secondary widget area it constrains the width of the widget container in the Main widget area (in the footer) – any idea how to stop this happening?

Thanks