Hey Thanks Ryan
I think I will just go with option #2 given I'll have the source list with all allocated reference numbers so if a form auto responder email containing their unique reference number is forwarded by an initial recipient to someone else (which I see as very unlikely), I can cross reference if I have to.
Though, very briefly (bulletpoint will be fine) would you mind explaining the process in setting up the JS option to generate the unique reference number?
I understand I need to put the JS you provided into the <head> tag of the landing page.
What else is needed exactly?
Do I need to create another field so the unique number has somewhere to live?
Or would I create a Custom Data Card?
Do I then add the processing step in the form to populate the new field or data card?
Is that unique number then added to an email via a field merge?
I love learning about such options for future reference.
Thanks again for clarification?
René