Sending passport application documents without reference number

Hi there,

I had trouble to apply my first adult British passport after naturalisation.

They internally sent my data to the next stage before my identity was confirmed by my friend. And when I call them I was asked to send my support documents to Durham passport office. Then I send them via special delivery Royal Mail. However, I forgot to write down the application reference number on the envelope! Very stressful for it because the original certificate and my current passport. How can I do it now? Any suggestions? Many thanks!