The Ides of April by Mary Ray

When Senator Caius Pomponius is found murdered, most likely by one of his slaves, the penalty for all the rest of them is death. However not all of the slaves were arrested that day. The steward was missing and also the young Greek secratary, Hylas, managed to escape. He is helped by a Christian, Varro, and by the son-in-law of the murdered man, a young tribune named Camillus. Hylas is desperate to save the other slaves, particularly his mother, and together they begin to solve the mystery but then Hylas, after finally being missed, is captured and Camilllus has to continue without his help. However he enlists the aid of the former consul and with his help he manages to solve the rest of the mystery and free the innocent slaves.