One For Sorrow John, The Lord Chamberlain Mysteries Series, Book 1.

In Byzantium, the capital of the 6th century Roman Empire, annual games are held to celebrate the founding of the city. Several courtiers, obliged by office to attend, idly watch the chariot races and the bear baiting from the imperial box. Suddenly they - and the crowd - are electrified as a magnificent bull surges into the arena. Those who worship Mithra make quiet reverance to the sacred animal while a trio of bull leapers enters in his wake....