For the sake of everyone following, I've come to this solution (most likely messy to any pro coders out there, but it works).
I edited the site's width to be 100% using the design styles. This meant however that the logo, nav links, and blocks of text also ran to the very limits of the page.
I therefore added margins using the custom css for the spacing of the logo and nav, and added in spacers either side of blocks that I didn't want running to the edge of the page.
On mobile view however, spacers disappear meaning that the text blocks ran to the edge of the page again. To override this, in the custom CSS I added in margin-left and margin-right values (25px), to bring the text away from the edge. I had to inspect the code to find out the div id's of the text blocks in order to dictate these margin values.