Mrs Carlisle got through life relatively unscatched, he says, but it's something he's still thinking about 40 years later.

"The letters scared the shit out of me if I'm honest," says McKinty.

"Some people thought they were a joke but I remember being petrified. I grew up in a part of Ireland that was quite superstitious. You just never knew what to think.

"These letters would have weird runes down the side, telling you your mother would die unless you made three copies and passed it on. You weren't going to take any chances were you?

"They were unnerving and scary and it's weird that it's stuck with me from primary school until now."

So much so that his latest standalone novel, The Chain, takes that concept to the nth degree.

Your child is kidnapped and to have them released you must pay a ransom and then kidnap someone else's child. And so the chain continues.

It's a diabolical novel, with twists and turns, asking the question of how far would ordinary people go to protect the ones they love.

When Rachel's 13-year-old daughter Kylie is kidnapped, the single mother, recovering from chemotherapy, is stretched to her limits trying to get her back.

"One of the things I wanted to ask of Rachel, she considers herself to be a good person, as most of us do, is how far would a good person go to look after their family, to take care of them," he says.

"And do you still remain a good person even though you've done really bad things, terrible things. I wanted to ask that philosophical question."

McKinty studied philosophy at Oxford University and talks about Aristotle's premise that says a virtuous person is only virtuous if they are doing virtuous things.

"If you consider yourself a good person, if everyone else considers you a good person, and then you go out and do bad things, then you're no longer a good person.

"I wanted the reader to go on that journey with Rachel because I still think of her as a good person even though, as she says, she becomes the monster.

"But to me she's not the monster even though she does monstrous things because it's understandable.

"I know I'm going to lose some people, some people are going to say, okay I wouldn't do that but the way I saw the situation, it was the only logical and necessary thing for her to do, I don't blame her for the things she does."

McKinty has teenage daughters of his own Arwynn and Sophie, 16 and 12, and we talk about how easily people give up information about themselves on social media. The novel explores this idea, the shadowy criminal network who runs the "The Chain" exploits people's naivety, Rachel uses social media to track down the movements of her potential victims.

"I'm not on Facebook but my missus is I went into her account and the first thing that struck me was how addictive it is. People are posting all the time stuff that's going on in their lives. you can read their whole feed in just one click of a button. some people are literally posting every 20 or 30 minutes about stuff they were doing in their life.

"It's incredible that people are giving that amount of information away and thinking it will all be fine, that we live in this safe world.

"People are generally nice out there but then you reading something like [Truman Capote's] In Cold Blood about this nice home in the middle of nowhere, where the entire family was murdered and that was real.

"Maybe we shouldn't give away so much information on social media, it was a cautionary experience going on Facebook and seeing how much people are willing to give up of themselves, yes 80 per cent of the time things will be okay but there are going to be cases where that stuff happens."

He wrote the first draft of the book in Mexico City in 2012 after learning about the concept of exchange kidnappings , whereby a family member offers themself up as a replacement hostage.

McKinty says he also had two films running through his head when he was writing The Chain, Ransom and Taken.

"In Ransom Mel Gibson is a millionaire and he's got access to unlimited funds and he's not an ordinary person.

"Big Liam in Taken 'has a particular set of skills' - [and the Irishman does an admirable imitation] - but Rachel has no special skills at all, she's just an ordinary person.

"I thought it's got to be a lot more interesting for me as a writer to find someone who can't leap over a wall or doesn't know how to shoot a gun, doesn't know how to break into a house, who has to learn all these skills from zero knowledge.

"That's what we'd all be doing. I don't know how to break into a house, I can't think of a single thing I do that would be useful. I'm a terrible driver, I'm always getting lost, I'm 100 per cent sure I would drop a gun if I had one ... I have not one special skill."

His special skill is writing. His first novel Dead I May Well Be was published in 2003 and shortlisted for the Ian Fleming Steel Dagger Award for the best thriller of the year. His Sean Duffy novel Police at the Station and They Don't Look Friendly won the 2017 Ned Kelly Award.

As much as he owes to the Duffy series, about a Catholic policeman in the Protestant Royal Ulster Constabulary during The Troubles in the 1980s, McKinty says he much prefers writing standalone novels.

"In a standalone novel you the writer and you the reader have no bloody clue what's going to happen at the very end, everyone could die on the last page, you've no duty to do a sequel, you've no duty to do anything else.

"I've done this in books and it's been Hamlet and the stage is just littered with bodies, sorry guys that's the way the story ends, I can do whatever I want.

"Where in a series everyone knows you're not going to kill the lead, especially when your publishers announce you've just signed a new three book deal or something like that.

"I tried to kill Duffy off in book three and now I'm on book seven. I had this idea, he hated Margaret Thatcher and we all knew book three was ending at Brighton when the IRA blew up the Conservative Party conference.

"I thought to myself what a wonderful poetic death that would be for Duffy if he saved Thatcher in Brighton and then he's killed in the bombing, even better because he's this Irish guy from Northern Ireland, maybe they even think he's one of the bombers.

"So he dies saving this woman he hates and he's utterly dishonoured and disgraced, there's no more beautiful, poetic, ironic, epic death that I could possibly give this character. It was perfect.

"I was so excited when I wrote the ending, I shipped it off to my editor and she was raging, she said no way we're not doing this.

"I said I'll never be able to kill him again in such a poetic way, and she said tough.

"And now we have Duffy seven and spoiler alert - he doesn't die at the end."