There’s chaos in HMP Pentonville.

A piercing alarm alerts us to what prison officers describe as an “incident”. There’s a cacophony of slamming metal doors, keys jangling, and shouts and screams from inmates as officers race to see what’s happened. We run behind as they head to where the trouble is.

Cell doors and chipped painted white bars are just about the only scenery as we move through this chaotic and nerve-jangling environment.

A muffled walkie-talkie tells us it’s a case of self-harm. An inmate who’s been locked up for most of the day has carved “mum and dad” into his arm with a sharp object. A quick glance into the cell and the sight of blood. A prison officer crouches down, stemming the flow.