Hi UKDeek,
Thank you for posting in the community.
According to this article: Embed your Excel workbook on your web page or blog from SharePoint or OneDrive for Business - Office Support (microsoft.com)
It’s first recommended to make sure that the embedded workbook is a public workbook. And you may go to the SharePoint site to check if it is shared to Everyone or not. If not, please re-shared it to everyone and use the new created link to see the result. Like shown below:
If the issue persists, please also try to use another web browser to see the result.
Best Regards,
May