Ch Supt John Sutherland seems tired. It’s the last Friday before his holidays, and he’s still got several meetings in the afternoon, but his fatigue looks like it goes deeper.

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Even if you hadn’t read Blue, Sutherland’s memoir about his 25 years in policing, and didn’t know about the crippling depression that put an end to his operational career in the Met just over four years ago, you would still notice his low, measured, tired voice.

You would hear the relief when he talks about leaving ground operations: “You could offer me the contents of the Bank of England,” he says, “and I wouldn’t volunteer to go back.”

You would know that the job that made him was also the job that broke him and hear it in his words - the immense pride for the things he’s done, and boundless grief for the things he’s seen.

Not that you’d need to do any guesswork. Sutherland doesn’t hesitate to discuss his illness – the depression that overtook him on a seemingly random day after years of bottled up trauma and daily sustained stress.

The cumulative impact of the policing job is at the centre of Blue. Sutherland says: “I think this is the most challenging time for policing since World War II for a number of reasons, from austerity to politics. And the nature of our work means officers have no reprieve from even the most distressing cases.”

Sutherland says that, when he was depressed, it took him some time to figure out what was happening to him. “Lately, we’ve begun to see a much more open and compassionate conversation about mental health. But when it happened to me it was terrifying and disorienting and crippling, and I didn’t have a clue what it was, so I couldn’t talk to anybody at work about it.”

Does he think this unawareness, this unpreparedness to talk about feelings, had anything to do with being a man?

“Yes. When you take being a man together with being a police officer, there’s still an exacerbated sense that we’re the ones who’ll get a call if somebody needs help. So the combination of my gender and job, in a culture in which mental health wasn’t widely understood was a huge factor, yes.”

Perhaps it’s this personal experience with accumulated trauma that gives Sutherland such a clear perspective on one of London’s worst problems: youth violence.

Sutherland has been stationed in seven London boroughs, amongst them Islington, where he was Operations Superintendent from 2008 to 2010. His big concern for many years has been knife crime, an interest that intensified after the 2008 murder of Ben Kinsella just off Caledonian Road, where the sixteen-year-old was stabbed to death by three youths.

Ben Kinsella’s death compelled his family to successfully campaign to increase minimum sentences for knife murders from 15 to 25 years. For Sutherland, Ben’s murder acted as a catalyst for deep reflection on the causes at the root of knife crime:

“There’s a connection between violence and street violence. Nine out of ten times, the youths involved in knife crime will have experienced violence at home: so they know no different.”

Two of the young men involved in Ben’s death had frequently witnessed their father trying to murder their mother.

“Violence begets violence,” writes Sutherland in one of the book’s most devastating chapters. In order to get to the bottom of serious youth crime, he says, “ask [the perpetrator] about the violence he’s witnessed and experienced growing up. All roads lead to home.”

Modern society’s frantic search for quick fixes is a topic that Sutherland comes back to time and time again.

“We live in a culture that’s impatient; we want fast broadband, fast food, fast public transport, and we get frustrated if we have to wait,” he says.

“That cultural impatience plays itself out in society, politically and institutionally – but, for me, youth violence and domestic violence need a 25-year plan. Because, as with many things, when you’re dealing with things that have been a generation or more in the making, we’re kidding ourselves if we think we can fix it by next Tuesday.”

Blue: A Memoir is published by Weidenfeld & Nicholson (£16.99)