A Change of Circumstance by Susan Hill (Simon Serrailler #11)

Simon Serrailler finds himself in devastating new territory as a sophisticated drugs network sets its sights on Lafferton and the surrounding villages 
DCS Simon Serrailler has long regarded drugs ops in the Lafferton area as a waste of time. Small-time dealers are picked up outside the local secondary school, they're given a fine or a suspended and away they go. And rinse and repeat. But when the body of a young drug addict is found in neighbouring Starly, the case pulls Simon into a whole new way of running drugs. The foot soldiers? Vulnerable local kids like Brookie and Olivia, who will give Simon a bitter taste of this new landscape...