I urgently need to achieve the following:

1. A document with "some kind of field" in the Header into which a unique number could be entered (either manually or by Mail Merge) - an example of the number format would be QCMR-10-1234, with the last 4 digits being the unique part.

2. This number must appear on every page.

3. The Form must be protected (pasword not required) so that users cannot accidentally modify the contents of the document, only the number field.

HOW COULD THIS BE ACHIEVED?
Your responses will be most welcome.
Best Regards,
James