Hello
I have managed to work out what is needed through a flash of inspiration. The first step is that of moving the text widget across to the page sidebar widget in the widgets section of the dashboard and then entering the desired header and additional content into the text areas provided. On editing a chosen page, one can use a drop-down menu on the right of the page to select the page layout and decide whether or not that particular page should have a sidebar. If a sidebar is chosen, then the text entered in the sidebar widget will appear for the given page. Unlike a different theme that I use, text inserted in the sidebar widget does not appear by default on all pages. I hope this information is helpful to others!