Passport application: Returning supporting documents to the passport office

[b1c]

Hi all

I had applied for my first British passport around mid-July. I had sent them my British citizenship certificate and my EU passport. The documents were received and my application was in the processing stage. I had however to ask for my passport to be returned back to me. Hence they sent me the EU passport only and kept the citizenship certificate. Thats fine, no problem with that. My application was put on hold until they have the passport back again
I want now to send them my EU passport so we can continue my application process. Does anyone know how this is done please? I mean just putting my passport in an envelope and then send it together with some letter maybe stating the application ref number and the explaining why I am sending it, is this going to be fine?
Is there some fee that I need to pay?
I am trying to call the office but the queue is more than 60mins long!

[HCJNL]

You have paid for your passport. I am not aware of any extra fees following such a request for the return of documents. So a letter with application references, and cover letter explaining why, is fine. You may wish to consider some form of recorded delivery.