An off-duty adventure of Biggles and his comrades of the Air Police at the hottest spot on Earth, sometimes known to members of the fighting services as Hell's Gates

goodreads summary

Biggles is visited by an old friend, Squadron Leader 'Dizzy' Digswell, who has an interesting tale to tell. After being forced to parachute out of his plane flying over the Aden Protectorate in Arabia, he thinks he has come across the ancient ruins of the city of Ophir. He found a cave, which he believes to be filled with ancient coins of gold and silver. Biggles and the team decide to form a syndicate to go and recover the coins