Thankyou for your quick response Mia,
I have tried your method with varied results depending on which computer I view it from and when.
At home and at work (UK based) the embedded Excel is visible without signing in on the host website via the iframe.
However I open an Incognito Chrome window on the same computer at home and the 'sign in' prompt is visible.
https://snipboard.io/QH0U5e.jpg
I have asked colleagues around the world to try, and results vary - some can view the Excel fine, some are presented with 'sign in'.
I have been careful to create a new webpage and new iframe solely to test your method, to ensure it is not a caching issue, and can confirm via the page source that the iframe link is updated to the one you have helped me craft.
If we browse to the direct 'anyone can view' link when it is not embedded in an iframe, then it works on all computers.
I wonder therefore if it is some security filtering based on the viewer's IP / location that is causing SharePoint to prompt for Sign In when a document is embedded in an iframe?
I do not want to post the website and link publicly because the data is not proofed yet but I am happy to share via private message.
Thankyou for your help