When you see The Penguin Lessons at the cinema, you might be surprised to learn it’s based on a book written by an unassuming chap who lives on Cornwall’s Roseland peninsula. But you won’t be nearly as surprised as Tom Michell is at the success of his book, which is based on his own experience and grew out of bedtimes stories told to his young children.

“My whole life has been full of surprises,” he muses. “My wife told me I should write it down; then I self-published on Kindle in 2015 because friends and relatives were being very kind about it, and I was fed up with printing it out on my own paper.

“Within a week, I’d had an email from Disney’s Animal Planet channel asking if they could make a programme; and within another week, I’d had an email from Penguin. The book has since been published in 24 languages, which is just astounding.”

It is the source material that is perhaps most incredible of all. A comedy-drama starring Steve Coogan, The Penguin Lessons sees Tom working as a teacher at an Argentinian boarding school in 1976, at the height of the brutal Peronist regime.

On a sightseeing visit to Uruguay, Tom came across thousands of penguins killed by an oil slick. He rescued one from death’s door, cleaned him up and tried to release him, only to find the penguin “stuck to me like a limpet”. Tom took Juan Salvador - the Spanish name for Jonathan Livingston Seagull - through customs and back to school, where he became an instant avian star.

“First the pupils loved him, then the staff, then the parents – they were all fascinated and made a beeline for my flat,” he recalls. “They would talk to him as you would to a dog or a cat, as if he understood.”

The surprises kept on coming. To Tom’s delight, Bill Nighy was chosen to read the audiobook. The film rights were sold in 2016, with The Full Monty director Peter Cattaneo at the helm and Philomena screenwriter Jeff Pope on script duties.

Tom sees clear parallels between The Penguin Lessons and The Full Monty. “The Penguin Lessons has the platonic love between person and penguin in the foreground, with military dictatorship stamping on the neck of Argentina in the background,” he explains. “That’s rather like The Full Monty, with its backdrop of the miner’s strike in the 1980s. It made me think Peter would do it well.”

They went out to Argentina to do some location scouting – then Covid hit. “I thought the project had died a death,” says Tom. In fact, it was simply a new beginning. Pope had filmed both Philomena and Stan & Ollie with Coogan, and asked Tom how he felt about the 59-year-old actor playing the lead role.

“The book is about me as a young man with no responsibilities,” says Tom. “I decided that the story about the penguin was unequivocally mine, but why should I object to one of the best actors in the country being in it?” A rewrite was in order, to give Coogan a back story befitting of a man of his age.

Tom also approved of Cattaneo’s portrayal of the political upheaval in the background. The brutality with which the regime disposed of its opponents is well-documented; the film is classified 12A, with violence implied. “In the book, I describe what’s happening, but on the screen, you have to make things happen. I was especially pleased to see the incredibly brave mothers of the ‘disappeared’. I take my hat off to them,” he says, with feeling. “They didn’t get their children back, but they were part of what brought down the military government. I’m proud the film pays tribute to them.”

Nine penguins were used in filming – one as the principal actor, the rest to keep him company. “Penguins are gregarious creatures.” These were backed up by animatronic birds almost indistinguishable from their real-life counterparts – but less messy. “Juan Salvador never came into my flat. Penguins crap everywhere,” says Tom, bluntly.

Upon his return to the UK, Tom was offered a teaching job in Cornwall, and grabbed it with both hands. “Sailing was the main draw - like Ratty, I love messing around in boats.”

Celebrity aficionados of the book include master of anthropomorphism himself, Paddington author Michael Bond. “When someone of consequence says they like it, I’m simply delighted,” Tom enthuses. “It’s tremendous, and I really didn’t expect any of it.”

This is characteristic of his modesty. “My role in it all is nothing more than chronicler for Juan Salvador - I’m Dr Watson to his Sherlock Holmes,” he insists.

“I only wrote the book because my wife said I should get the stories down for our children. Then there I was, standing on a beach in Barcelona with Steve Coogan, with him giving me a hug and saying how much he was enjoying being me.”

Ask Tom what he’s most proud of regarding The Penguin Lessons, and he answers without hesitation: “Out of all the books they could have chosen, South Korea put mine on the national curriculum for 15- and 16-year-olds. It’s astonishing. It’s the highest accolade I could have achieved – the pinnacle.”

He hopes the film will raise awareness of the plight of penguins and other sea life today, blaming a dramatic drop in numbers on overfishing in the south Atlantic. “It doesn’t take a rocket scientist to work out what that means for the creatures that rely them for food,” he says. And there is still pollution. People are still rescuing penguins covered in oil, and sea lions caught in fishing nets.

“I hope the film makes people think about turning down the central heating and putting on a sweater, and being more careful about fossil fuels. If they draw lessons from it and find it meaningful, that’s wonderful. But,” he adds, “if they come away thinking, ‘that was a fun film’, that’s equally fine by me.”

The Penguin Lessons (12A) began playing at cinemas across Cornwall from April 18.