When I first call Ros Hemmings, I expect her to be surprised. A widow in her 60s living in rural North Wales, she has never received a call from an investigative journalist in London. But instead she tells me: “Oh no, I have a very good idea why you are ringing.” She has been waiting for this call for years.

Hemmings rightly suspected that I wanted to ask her about a woman she knows as Sally Walker. Millions of people around the world know Sally by a different name: Raynor Winn. She is the author and protagonist of one of the most successful British non-fiction books in recent years. The Salt Path traces Raynor and her husband Moth’s 630-mile journey along the sea-swept South West Coast Path.

A heartbreaking “true” story of two people in their early 50s forced out of their rural home in Wales and weighed down by a sudden diagnosis of Moth’s terminal illness, The Salt Path went straight to the top of the bestseller charts, selling more than 2m copies worldwide since its publication in 2018.

Winn has since written two sequels and has a lucrative publishing deal with Penguin to produce at least one more. Five weeks ago The Salt Path reached new audiences when it was released in the UK as a film, starring Gillian Anderson and Jason Isaacs, and Winn is a co-producer.

Standing proudly on the red carpet outside the Lighthouse Cinema in Newquay, Raynor, 60, told TV cameras at the film’s UK premiere that the experience was “almost unbelievable”. In that moment, she and Moth seemed like the ultimate examples of British grit and perseverance.

Back in Wales, Hemmings saw a very different picture. Because she knew something about Winn that almost everyone – her publishers, her agents, the film producers – had missed. She knew that Raynor Winn wasn’t her real name and that several aspects of her story were untrue. She also believed she was a thief.