Thanks Jochen.
Unfortunately, giving Anonymous, Reader access, doesn't work.
I have created an HTML form, which when submitted, creates a document in the Domino database. If I set Anonymous as depositor, then the form is submitted and is accessible via the automated email link, until I use the $PublicAccess flag you suggested.
If I set Anonymous to Reader, it prompts for Username and Password when trying to submit the initial HTML document to the Domino database.
Maybe I will just stick with Depositor access.