The page was being held to the left by the existence of the sidebar.
You could use CSS to change the right margin, and you could also increase the width of the post to take up some of the available space. It depends on what you wan’t from a design point of view.
Thanks for pointing me in the right direction Adrian.
I added the following code to my child theme css template and it worked fine:
/*Adjust alignment to the left on data tables */
.full-width .site-content .entry-header,
.full-width .site-content .entry-content {
margin-left: 0;
margin-right: 50;
max-width: 100%;
max-height: 100%;
width: 950px;
}