Do I need a multiple entry visa?

I am travelling to Beijing on business and have arranged a single entry one. While there I am taking the opportunity to visit my daughter in Cambodia. I fly out of Guangzhou on the 20th March. I fly back to Guangzhou on the 24th March and then catch a flight from Guangzhou to London on the 25th March. Do I need a multiple entry one or is my second visit to China eligable under the 72 hour free transit scheme?