Hi Kathryn, thanks for answerring. I managed to solve my problem, after finding another asnwer and tutorial. Easy once you know how!!
I created a child theme, created a new page template for that page (refers the page ID), and used CSS to copy and amend the “main” id which that template refers to.
Im assuming if I add further full width pages, I can refer the new template to a page category. Parament doesnt seem to have options for template or category in the editor page, but I have found a plugin that adds categories to pages.
Thanks for your interest – my site is nzpersimmons.org.nz