Per the instructions I've created the CNAME file
In the "Code and automation" section of the sidebar, click Pages.
Under "Custom domain", type your custom domain, then click Save. If you are publishing your site from a branch, this will create a commit that adds a CNAME file directly to the root of your source branch. If you are publishing your site with a custom GitHub Actions workflow, no CNAME file is created, so you need to create one manually (containing only a line of text with your custom domain). For more information about your publishing source, see "Configuring a publishing source for your GitHub Pages site."
However, creating the CNAME file didn't seem to do it. It made no difference and I'm still getting the 404.

