The Terror of Constantinople (Death of Rome Saga Book Two) by Richard Blake
If you loved Gladiator and Spartacus, you'll love the second book in the DEATH OF ROME SAGA.
Invaded by Persians and barbarians, the Byzantine Empire is tearing itself apart in civil war. Phocas, the maniacally bloodthirsty Emperor, holds Constantinople by a reign of terror. The uninvaded provinces are turning one at a time to the usurper, Heraclius.
Just as the battle for the Empire approaches its climax, Aelric of England turns up in Constantinople. Blackmailed by the Papacy to leave off his career of lechery and market-rigging in Rome, he thinks his job is to gather texts for a semi-comprehensible dispute over the Nature of Christ. Only gradually does he realise he is a pawn in a much larger game.