As a police hostage and crisis negotiator John Sutherland saved dozens of lives during his 25-year career — but he is still haunted by the one negotiation he could not resolve.

The former senior Metropolitan Police officer, who has used his experiences to write a thriller series starring police negotiator Alex Lewis, spent five hours trying to persuade barrister Mark Saunders to put down a shotgun he was waving out of the window of his Chelsea home in 2008.

The siege in Markham Square ended with the 32-year-old being shot at least five times by police marksmen and killed.

John, 53, promised the dead man’s wife not to speak about the details of the negotiations but admits the loss of Mr Saunders’ life is one of a handful of cases that left an “indelible mark” on him.

There are 84 hostage and crisis negotiators in London – 76 in the Met and eight in City of London Police – and far fewer in police forces across Britain.

Most officers carry out the role on a voluntary basis alongside their “day job”, and for no extra pay.

John, who reached chief superintendent as the borough commander for Camden and then Southwark, feels the public do not really understand the role, as the sensitive nature of negotiations means they are not widely spoken about or reported on.

This was part of his motivation for writing the Alex Lewis series. Its second instalment, The Fallen, is out now.

The book starts with Alex being called to help a young woman who is threatening to jump from Westminster Bridge.

Perilously perched on a narrow ledge outside the bridge railing, Becca Palmer, claims she was fired from her Home Office job after an affair with a married policing minister who she believes is corrupt.

Alex vows to look into her allegations as the best way of stopping her from trying to take her life.

John says unlike in Hollywood movies, where hostage negotiators are regularly called in when bank heists go wrong, this type of situation is very rare in real life.

Instead, he was mostly called to deal with people suffering with mental health issues or other traumas.

He says: “The vast majority of what you are called out to is people in crisis. Just ordinary, decent people with no criminal history who for a thousand different reasons have ended up at the end of themselves.”

John attended a care home in east London where an old man was sitting in an armchair in the communal lounge holding a large kitchen knife to his throat. He says: “I sat down in a chair across the room from him and encouraged him to tell his story.

It turned out he’d fallen in love with another of the residents in the home who had not only spurned his advances but had been unkind to him.

“He was lonely, broken-hearted and wanted to be heard. In the end, I was able to persuade him to put the knife down. I’ll never forget his sadness.”

In another negotiation, John went to Merton town hall to find a man sitting in a consultation booth, on the other side of a reinforced plexiglass window, with a hypodermic syringe pressed into an open wound on his neck.

He was in dispute with a council housing officer and had snapped.

John says: “I can’t remember how long I spent with him, but he gave himself up in the end.” The father-of-three from Southwark says being a good listener is the most important skill for the role. As a dedicated and caring officer, this natural empathy was an asset.

But John – who will be appearing at Orion’s Incident Room at the Theakston Old Peculier Crime Writing Festival in Harrogate on July 21 – admits it also probably played a part in suffering a mental breakdown in 2013, aged 43. He says: “I broke into a thousand pieces. I ended up off work for more than seven months.”

John made it back to work for more than five years but was not able to return to full duties, so was medically retired in 2018.

His involvement in four tragic knife crime deaths has also stayed with him.

For this reason The Fallen is dedicated to Kodjo Yenga, Ben Kinsella, Milad Gol-makani and Dogan Ismail.

John says: “They were in different parts of London at different times but along with some of the domestic violence cases I dealt with, and along with some of the crisis negotiations I was a part of, these are the things that have left an indelible mark on me.”

While John was off work he started writing, at first just to exercise his brain. But he said he soon found it cathartic.

“I started writing about the things I had done and gave myself permission to actually feel them for the first time.

“That was both frightening and painful but also enormously healing.

“I have kept writing because I love it and I feel incredibly fortunate to be able to live a second life.”

• The Fallen by John Sutherland (Orion, £18.99) is out now. Visit expressbookshop.com or call 020 3176 3832 to buy a copy. Free UK P&P on online orders over £25
• The Theakston Old Peculier Crime Writing Festival takes place July 20-23. Visit harrogateinternationalfestivals.com