Teasing, I ask how he gets on with Oliver’s mother, Jo Gideon. Osman describes himself as a lefty and she is the Tory MP for Stoke-on-Trent, a red-waller instrumental in ousting Johnson. It’s the only time Osman looks terrified. He doesn’t lie. Instead he says: “Let’s not go there. We won’t go into that.” In a peaceful retirement village, four unlikely friends meet up once a week to investigate unsolved murders. Some of the best fictional sleuths are older and wiser – from Miss Marple to Columbo to Jessica Fletcher in Murder, She Wrote – but he cannot abide the term “cosy fiction”, words he introduces to the conversation and has a severe allergic reaction to without any input from me. “If you really read my books, there’s some quite bad stuff happening, some very non-cute references. It’s definitely not cosy. Today you can write a book about a detective who runs a sweet shop in a seaside town and someone will publish it. But that’s OK,” he says moving from the heat of his own irritation. “I get it.” I’ve written my whole life. Graham Norton has always written, Dawn French has. It is not a surprise that these people go on and write books. You’re allowed to He’s described his 40s as “really good fun.” He was single for much of it and there was a merry-go-round of dates. “I was always looking for the one, always knew I wanted to get married, absolutely wanted to fall in love. And, listen, I enjoyed the process. Friends would go, ‘I don’t think that is what you’re looking for. I think you enjoy playing the field.’ I would always say, ‘It really is what I’m looking for.’”

Coopers Chase, the fictional retirement village where his characters reside, is based on the community in Sussex where he bought a house for his mother, Brenda. He looked around and immediately realised the potential. Here was the generation who, culturally, “are overlooked by everybody. That generation did much more interesting and unusual things, overcame much bigger hurdles and obstacles. It’s a generation full of wisdom, full of brio, looking for new adventures and new mischief. There are very few consequences to anything they do or say. That’s freeing for a writer, to have characters who are going, ‘No one’s going to arrest me; I might as well do this.’” They are overlooked because we worship at the temple of youth, he says. “I mean, God knows what our generation will be like when we get to that age; insufferable.”Photographer’s assistant: Caz Dyer. Set Design: Sandy Suffield. Assistant: Lucas Aliaga-Hurt. Grooming: Pauline Simmons. Photograph: Jay Brooks/The Guardian The Last Devil to Die is no exception. It’s a crime story, yes. But at core it’s a book about dementia and assisted dying. Where his mother lives, residents are over 75 and, “They had a big debate about it, incredibly rational, incredibly polite. Lots of disagreement, [but] everyone listening to each other. People who have been medical professionals, people who’ve been mental health professionals and people who’ve obviously lost loved ones. It’s something that you’re allowed to talk about. It’s not crazy to want to die when you’re in pain with no way of getting out of that pain. I absolutely respect the views of people for whom [assisted dying] would be an impossibility. But it’s an argument that’s not going away. We have such control over our lives, it seems weird that the final bit we have no control over. An awful lot of people would sleep easier if they knew their last few years wouldn’t be very difficult.”

Osman always loved crime. He grew up reading Christie and adored Patricia Highsmith, creator of Tom Ripley. He also likes the peculiar Britishness of the worlds created by Barbara Pym and Muriel Spark. It didn’t feel like a jump to write books, having written for TV. Although he has been accepted with open arms by the crime writing community, there is still a trace of the testiness he felt over an early suggestion that he is one of a slew of celebrities turned authors. “There’s certain books that come out and people are open about having a ghost. I get that people know what they’re getting and understand it’s a brand. But there’s also a group of people – Bob Mortimer is one – where we’re just writers. I’ve written my whole career, my whole life. Graham Norton has always written, Dawn French has. It is not a surprise that these people go on and write books. You’re allowed to. Also: no one is a writer. Everyone is something, then becomes a writer. You get to a certain age and think, ‘Well, I want to write a novel. I’ve got stuff in my head that I want to say.’ No one ever buys a second novel if the first one isn’t good.” I would have been a terrible spy. I’m too tall, not bright enough, and if I have a secret, I tell everybody. I cannot tell a lie Osman watched dementia take possession of his working-class firebrand grandfather, watched him try to cling to moments of clarity. His mother told him that in hospital you could see his heart beating and knew it was never going to give up. “He was such a strong man. But he would absolutely not have wanted to be there.” Osman drew from this experience and also research. “The Alzheimer’s Society said, ‘If you’ve met one person with dementia, then you’ve met one person with dementia.’ That’s how I approached it really: knowing that everyone’s experience will be different.” Whether work was going well, or his marriage was going badly, there was another issue nagging away. In his 30s he had an anagnorisis of sorts: went to therapy and was diagnosed with addictive behaviours, the most explicit around food. For years he’d suffered bouts of binge eating, “an absolute compulsion to eat, an inability to stop eating, shame afterwards and then repeat”. The pattern could continue for weeks or months. “I find myself in situations sometimes where my behaviour around food is so absurd, it makes no sense. It’s certainly not self-care.”Does he remember what age he was when it started? “Oh, like 10 years old. Yeah, I wonder what the inciting incident was. And food when you’re 10 is something that you can’t control. You’re not going to become an alcoholic or a drug addict.” While it doesn’t have the “doomed glamour” of alcohol or drugs, he has said, the behaviour is in essence the same – although “slightly more behavioural and slightly less to do with the substance itself”, as with love or sex addiction. “But the second you go to therapy, you realise that’s just a symptom of the problem. You realise you’re just numbing whatever pain; you’re numbing the things you don’t want to think or talk about.” I’d buy this more entirely if it weren’t for the faintly barbed quote Brenda gave to the Times about his writing style being “quite staccato”, which suggests she has no qualms chivvying her son. Either way, it paid off. Osman says he was probably the first from his school to go to Cambridge, where he did sociology and politics (although he still regrets not doing American studies at Leeds); his brother, “who is proper clever”, did economics at the London School of Economics.