The phone in my hand buzzed like an angry wasp. I was sitting in an unmarked police car in a north London side street, across from a bustling café. It was our informant “Chucky”, a stocky, middle-aged man who’d been on the fringes of the capital’s crime scene for years. He was widely known and well-respected, charming and funny.

“He’s just called me, he’s sitting inside,” he told me in his estuary English accent.

“Okay, give it five and then go in,” I said.

“One of ours is already inside having a brew.” “Nice one, Geezer.” He rang off.

“Stand by, stand by. Target’s in the café now, sitting at the back,” I said into the radio in my other hand. A number of rapid clicks was enough to confirm our officer inside the cafe, Jeff, had heard and understood.

Apparently engrossed in his newspaper, he was the eyes and ears of the police team inside the café.

I was a detective constable on a proactive crime task force in London, and case officer for this deployment. I’d done the research, come up with the plan, pulled in all the resources and put the job together. All we had to do now was wait.

Chucky had heard a whisper the man in the restaurant was touting for someone to commit a murder.

Scant details were available, no idea of targets or timelines, other than it had to be done soon. Nothing was known about the broker.

Chucky was now going into the cafe. “Our man now entering the restaurant, all units wait,” I reported into the radio. More rapid clicks erupted from the speaker as Jeff indicated Chucky and the broker were now sitting together.

“You ready?” I asked my partner. He nodded, grimly. I spoke into the radio again: “All units, strike-strike-strike.”

Two liveried Territorial Support Group vans screeched to a halt outside the café, and two lines of cops, hats pulled low, gloves on, stormed in, quickly and efficiently. “That’s two detained, both in cuffs now,” came the voice of the TSG team leader.

I smiled as I thought of poor Chucky, now in cuffs alongside the broker.

He would be dealt with as a prisoner, meaning a night in the cells, but he wouldn’t mind. He knew there’d be a payday in it for him, and it gave him plausible deniability.

I sent my partner to “arrest” Chucky whilst I went to see the broker.

He was a small, wiry guy, with a shaved scalp and goatee beard. Despite being cuffed he was the picture of calm.

He turned his gaze to me, appraising me with icy blue eyes.

“What’s your name, Pal?” I asked. “Lekaj,” he said, his voice thick with the tones of Eastern Europe. “Where are you from?” “Albania.” He had been arrested with two envelopes on him, one packed full of cash and the other with two names and addresses scrawled on the back of it – the victims for the Killing Eve-style hit Lekaj had been trying to set up.

He held my gaze, his face impassive.There was something in those icy eyes I hadn’t seen before. A calm, assured ruthlessness that was unnerving.

Back at the station, we traced both men from the names on the envelope. They seemed to be innocents, uninvolved in any criminal activity.

One was a middle-aged man living in the south of England, the other a small businessman in the Yorkshire dales. They had never been in contact with each other, and neither had ever had any trouble with the police. So why were they on a “hit list” in the pocket of an Albanian fixer?

Then, we found the link. Separately, they had both been involved in minor disputes with a third party. One had resulted in small claims court action. The other had provided a service to the same individual which had ended in a row.

That was it. Very minor disagreements with the same person had resulted in two contracts to murder. It seemed unbelievable, but the evidence was strong. The suspect who ordered the hits – I’ll call him ” Mr X”, described as “flash, brash and unpleasant” – had taken serious offence in both of these almost entirely innocent interactions and become threatening. Lekaj’s customer turned out to be an internationally-based career criminal originally from the Mediterranean, but a British citizen for many years.

He was a successful fraudster and moneye launderer for drug trafficking gangs. And he was already gone.

Having left the UK some months before, he was now living in a hot country that had no extradition treaty with the UK.

A few days later, I met with Lekaj at a police station when he came to collect some property. The Crown Prosecution Service had decided we did not have enough evidence to proceed with a prosecution for soliciting murder. It was not a surprise.

Lekaj was cheerful and actually quite charming, probably because he knew we had nothing we could use.

“So how did you come to arrest me?” he asked, with a half-smile.

“You want to be careful when and where you talk, mate,” was all that I said.

His eyes narrowed, but the smile didn’t completely disappear.

“You know those people on the envelope are innocents, right?” I said.

Doubt flashed across his face, so I decided to press home.

“Total innocents. Civilians. Just people that fell out with Mr X. Not bad guys at all.

Married, kids, all of that stuff.” His face was flat, but his eyes told their own story. When he spoke his voice shook with emotion. “That’s not what I do, Mr Lancaster.

“I don’t touch innocents. I was told they were bad people.”

I could see he was disturbed. I’d clearly touched a nerve and was shocked to see tears begin to well up at the realisation he had nearly been duped into killing two ordinary people who had upset the wrong man.

“I’ll say this once only, Mr Lekaj,” I warned, staring him straight in the eyes. Anything happens to either of them and we’ll know where to look.Whatever country X is in, it won’t be far enough away. We’ll catch him, and we’ll catch you, you understand me?”

“Understood,” he said. We shook hands and he left. Despite the absence of any successful prosecution, we considered this case there at to be a success. A potential double homicide had been disrupted, no real-life Villanelle-style contract killer had been set loose, and two innocents were still alive. Not all police operations result in prosecutions, and this is a perfect example of one which failed to make a courtroom.

This account is based on a true case. I’ve changed some details to protect the innocent, (and the guilty), but it happened. The targets of this contract never heard from Mr X nor any of his associates again. They carried on with their lives, although I suspect they continued to look over their shoulders.

The public are generally blind to incidents like this, although they happen day after day, week after week, across the UK.

Threats to life are received regularly, and small teams of officers are called in to investigate and disrupt. Most never make the newspapers or end up in court, coming as they do from sensitive intelligence sources.

I spent 25 years policing London but I’m not sure if anyone unnerved me more than that Albanian broker.After I retired from the Met in 2015, I moved to the Scottish Highlands and took the plunge by becoming a thriller writer, leaning heavily on my former career for inspiration.

Cases like this have been invaluable as source material for characters and fictional incidents I can make authentic.

My latest book, Dead Man’s Grave, features Scottish gangsters using contract killers to settle a centuries old feud – and involves a lot of undercover work.

Such policing undoubtedly had a bad reputation. Cases like that of Colin Stagg, an innocent man initially suspected of the murder of Rachel Nickell, 23, on Wimbledon Common in 1992 and targeted by an undercover police woman known as “Lizzie James” who, it was hoped, would tempt him into confessing by feigning a romantic interest in him.

Or the undercover police officers who infiltrated animal rights groups in the 90s, entered into relationships with activists and even fathered children before disappearing.

These cases are aberrations. Most successful undercover operations never even come into the public domain.The priority of policing is always preservation of life above all else and, because of this, occasionally criminals go free.

However, there’s an old saying in policing, “He’ll come again,” and in most cases, they will. Do enough bad stuff, and eventually bad things happen to you.

Now aged 55, I live a tranquil existence. I loved my time as a cop. It was interesting and fulfilling and it gave me a lifetime’s worth of experiences to help me create stories that people seem to like reading. But somewhere in the UK right now, police officers are out there, in the field, putting themselves at risk for one reason only: to protect the public and save lives.

They’re brave people and they work tirelessly to do the right thing. The chances are the public will hear nothing, they’ll receive no thanks, and yet they’ll keep putting themselves in harm’s way day in, day out, to make our country just that bit safer.

We should all be thankful.

• Dead Man’s Grave by Neil Lancaster (Harper Collins, £14.99) is out now. Call Express Bookshop on 020 3176 3832. Free UK P&P for orders over £20