Clarification Please regarding 72hrs Entry twice in a week

Thanks in advance but I would be greatful for clarification on entry into Beijing as I have been given conflicting advice. My itinerary is as follows:
London - Beijing. Stop over 48hrs in hotel
Beijing- HOng Kong. 4 Nights.
Hong Kong -Beijing. 12 hours in hotel overnight.
Beijing-London.
All on a Full British passport.

Pleas can someone confirm if I require any sort of entry permit due to 2 entries into Beijing. Also what is the procedure on arrival into Beijing, do you go to customs or have to visit a Visa Desk first and what paperwork is required.
Many Thanks