The old adage of never working with children and animals is being tested by ITV with its main Sunday night drama. But The Durrells, a fun mix of gorgeous locations, four badly behaved kidults and an odd menagerie of animals, is proving a success for the broadcaster which has been searching for a Sunday night hit since Downton Abbey came to an end in December.

Based on Gerald Durrell’s Corfu trilogy, the drama starring Keeley Hawes as the whimsical widow fleeing a prewar Britain for the sunny Mediterranean is attracting an average audience of 5.9 million viewers on a Sunday night, nearly 2 million more than the channel’s average.

The series has also been a boon for sales of the the naturalist’s books. Waterstones said sales of My Family and Other Animals, the first of the Corfu trilogy, had more than tripled, while sales of the trilogy had increased eight-fold.

Publisher Penguin Random House said it had seen an “incredible uplift” across all three editions of My Family and Other Animals.

“In our experience, successful book adaptations for TV and cinema often lead to a renewed interest in the original book,” said Richard Humphreys, Waterstones’ non-fiction buyer. “It is an amazing result and a testament to the brilliance and longevity of Durrell’s writing.”

Shown in a pre-watershed slot of 8pm, the combination of romance and rollicking children has won over entire families. The Durrells is the highest rated new drama series on ITV since Cilla in 2014 and was recommissioned after just two episodes.

Sally Woodward Gentle, the executive producer, said the idea of turning Durrell’s three books into a long-running series came about because of the commercial broadcaster’s demand for a family-friendly hit on the biggest night of the week.

“We wanted sunshine and escapism, and not anything too slight or sweet or patronising,” she said. “We don’t want to be too sickly sweet about it. Anything that feels a bit cute we will try to undercut it. At the same time, it has to be something that’s incredibly pleasurable and a joy to watch.”

The appeal was ageless, she said, because the show contained “quite naughty children who are allowed to run wild and lots of animals. For the adults, the wit and characters are amazing”.

Conceived as a long-running series, minor characters such as Sven and the part-time prisoner Costi are made much larger while the central character of Louise is given a possibly more modern spin. “She’s a single parent, feisty and bolshie, and does this extremely brave thing by taking the children off to Corfu,” said Woodward Gentle.

There have been comparisons to the the Larkin family in the early 1990s hit Darling Buds of May, which Woodward Gentle welcomes. “I loved it,” she said. “It was slightly naughty and all about how lush nature is.”

Critics, often less enamoured of ITV drama than they are of BBC adaptations, have been generally positive, although there was also criticism of a plot twist dubbed the “gay switch scandal”.

Sven, who is gay in the book, becomes Louisa’s main love interest on ITV, prompting charges of “straightwashing”.

ITV would not be drawn on how the storyline between the two plays out, with the penultimate of six episodes to be broadcast this Sunday.

Woodward Gentle said the show was very loosely based on the original books, and Durrell’s widow, Lee, had been very supportive. Neither the original memoirs nor the writings of Durrell’s older brother about the family’s time on Corfu was entirely accurate, Woodward Gentle said.. “Gerald and Lawrence Durrell didn’t tell the truth either … They didn’t let truth stand in the way of a good story.”

Written by Simon Nye, whose credits include Men Behaving Badly, the series hopes to stay true to the humour of the books.

They covered a four-year period before the onset of the second world war made the real-life Durrells return to the UK. With filming of the next series set to start this summer for the next series, Woodward Gentle said she would “love it to go on and on”.

Ratings war

The Durrells has helped to loosen the BBC’s grip on early Sunday evening viewing, which is traditionally dominated by Countryfile at 7pm, followed an hour later by Antiques Roadshow, both on BBC1.

Last Sunday, The Durrells had 5.5 million viewers from 8pm, just ahead of Antiques Roadshow with 5.3 million.

ITV’s previous attempts to find a hit pre-watershed drama on Sunday sank without trace. Jekyll and Hyde last year and Beowulf earlier this year were both dropped after ratings fell to 2 million viewers.

Among the other big Sunday night dramas, BBC1’s Undercover, with an overnight audience of 4 million viewers at 9pm, has not managed to repeat the success of The Night Manager, which gripped more than 6 million people in the same slot. But neither were in the league of the biggest Sunday night drama hit, BBC1’s Call The Midwife, which at its peak was watched by 10 million viewers.

Channel 4’s Indian Summers has been a casualty of the fierce Sunday night ratings war, with the drama about the birth of modern India axed after two series after its audience fell as low as 1 million.