When do I need a copyright declaration from users?

You need copyright declarations when providing articles under "library privilege" (fair dealing).

This declaration is to say the user has not previously had a copy supplied, that the copy is required for non-commercial research or private study, that they won't supply the copy to anyone else, and that as far as they know, no one else is going to be asking for the same copy for the same purpose at the same time.

You can only provide 1 article per journal issue, or 1 chapter/5% total of a book.
The declaration is stored securely by the library for 6 years.
You do NOT need declarations for content provided under the NHS CLA licence.