Beast of Burden (Cal Innes #4) by Ray Banks

The biggest race riot in Manchester's history and his brother's death have left PI Callum Innes a physical and emotional wreck. He's about to leave his newly established agency to partner Frank Collier when the word comes through that his erstwhile nemesis Mo Tiernan has gone missing. Innes is the only one Mo's ganglord father trusts to investigate, but he's not the only one working the case - Detective Sergeant 'Donkey' Donkin has a vested interest in both the Tiernans and Innes, and he'd sacrifice his career to see them both behind bars. After all, he has a score to settle...And he's not the only one.