Can I use the 72 hour visa transit waiver?

I am going to be flying into Beijing airport in August and be staying for 48 hours. Before flying to Pyongyang, for a 10 day tour, returning back to Beijing before taking a flight to Hong Kong in the evening of that day, staying there for three days before flying back to the Uk. Can I use the free transit for this trip? Who do I need to contact before I fly so they are aware I am on the 72 hour programme? What information will i need to take with me so I get the 72 hour waiver, where do I go to get it? Where is the desk in the airport where I get it from? Is there a fee for the 72 hour free transit? Any other information I need can you please update me so I am fully aware and accommodate for it.

Yours sincerely
Daniel North