Applying in Ulan Bator for a Chinese visa

Hello Sir/Madam
I am a UK citizen.
I have visas to travel across Russia and enter Mongolia by train.
Can I apply in Ulan Bator for a double entry tourist L to enter China ?
I intend to join a tour group in Beijing and travel to North Korea.
What information willI require ? Will confirmation of hotels be required for each days stay in China ?
How long does it take for it to be issued ?
What is the cost?
Many thanks.
Barry Jepson