“I like you very much,” he says (translation: “I am in love with you”). But Bridget fancies her boss, Daniel Cleaver (Hugh Grant), a charming scoundrel who promises mini­breaks and great sex.

In The Edge of Reason, the sequel, directed by Beeban Kidron, we find Bridget and Mark having a post-coital hug with his boxer shorts neatly folded by her bed. But this is not a happy conclusion to their tale. It’s more like the beginning of the end.

His formality starts to seem an awful lot like pomposity; his folded boxer shorts start to annoy her. She chucks him, escapes to Thailand with Daniel (who is now employed as a travel presenter by the television channel for which she works) and ends up being saved from a Thai prison by Mark.

Of course, without the right actors neither of the films would have worked. The combination of Zellweger, a Texan with an excellent Home Counties accent and a capacity for physical comedy (“a latter-day Lucille Ball”, according to one critic); Firth, haughty and smouldering (even in a home-made jumper with a reindeer on the front); and Grant, revealing a wickedly seductive darker side, won over audiences.

And now, 12 years later, we have the long-anticipated Bridget Jones’s Baby, again directed by Maguire, which rejoins the characters after 10 years.

Bridget has broken up with Mark, is a successful news producer and owns her flat. Daniel is a fond memory. But life hasn’t worked out quite as she’d hoped. Her old friends have settled down with babies (she’s now godmother to countless), and she rarely socialises with them. Instead, she sees her younger, single colleague Miranda (Sarah Solemani). They are united against their (even younger) boss, Alice Peabody (Kate O’Flynn), who wants a populist version of the news.

It is Miranda who drags Bridget off to a music festival, where she has a liberating encounter with a handsome American, Jack Qwant, played by Patrick Dempsey. Mark, meanwhile, is still a barrister, has developed a hard set to his mouth and is married to someone he doesn’t love.

He and Bridget meet by chance, spend the night together, then go their separate ways. A few weeks later, Bridget discovers she’s pregnant. But who is the father – Mark or Jack?

The story of Bridget and Mark has turned into a kind of cinematic novel, comparable to Richard Linklater’s Before Sunrise trilogy, in which the same actors, Julie Delpy and Ethan Hawke, reprise the same roles over the course of two decades.

Here we have Zellweger and Firth in a love story played out over 15 years and three films. The actors have grown older with the characters.

“The passage of time has added texture,” Firth says, when we meet in Los Angeles. “Time adds history, and it’s a collective history now. It’s not just us as actors, it’s all of it together.

“It’s us as characters, it’s the fact that Sharon was there at the beginning, as well. We’ve all had lives in the inter­vening years – 15 years have gone by for all of us, and it has taken its toll in some ways, and it’s also added to our lives. It’s brought regrets, probably for everybody. We all have those things, and it will have done that for the audiences as well.”

Bridget’s story still taps into universal themes, Maguire says.

“There are lots of women in their 40s who haven’t chosen marriage and children who are out there on their own. Her fear of loneliness is still a very valid fear. It is something we are all frightened of.”

As for why she wanted to reprise the role, Zellweger says, “I love this character and all the people involved. “So many people feel they can relate to her because they’ve shared her awkwardness and they’ve shared her humilia­tion. She makes it OK to be imperfect and we are all kind of looking for that.”

THE FIRST BRIDGET JONES column appeared in 1995. It was an instant success. With her attempts at self-improvement (listing calorie intake, units drunk, cigarettes smoked and efforts to achieve “inner poise”), jobs, clothes, love life, drinking bouts with friends Shazza and Jude, and her mother’s attempts to marry her off, Bridget laid bare the woes of 30-something single women in the 1990s (and launched such phrases as “smug marrieds” and “granny pants”).

“I got the idea for the calories, etc, when I looked at my university diaries, which had a sad lack of social engage­ments but these mad lists of calories: yogurt, 150; carrot, 15; box of Cadbury’s Milk Tray, 4,567, etc,” Fielding says. The plot and characters were lifted from Jane Austen. “I thought they’d been market-researched for centuries, and Jane Austen probably wouldn’t mind. Also, she’s dead.”

The book based on the columns, Bridget Jones’s Diary, came out in 1996, and together with the sequel, Bridget Jones: The Edge of Reason (1999), sold more than 15 million copies worldwide. Working Title Films bought the rights to the columns and the book.

“This was a character who just flew off the pages,” says Eric Fellner, co-chairman of the production company.

Fielding co-wrote the script for the Bridget Jones’s Diary movie and worked on the subsequent films – Bridget Jones’s Baby was written with Emma Thompson and Dan Mazer. Before the first film, Maguire had never directed a feature.

Yet she was uniquely qualified: a close friend of Fielding, she was the inspiration for Bridget’s friend Shazza – “a journalist who likes to say f*** a lot”, she says.

“Eric and Tim [Bevan, also co-chairman of Working Title] had imagined an indie, low-budget movie – maybe around US$2 million – so maybe a new director wasn’t going to be such a risk,” says Maguire.

“I think they thought, ‘She can’t mess it up that much.’ Then when Hugh agreed to play Daniel Cleaver and the movie began to become a more commercial prospect, it started swelling and swelling. Colin joined, then Renée, and the stakes went right up. But because it was incremental, I didn’t have the chance to go, ‘Oh holy f***, I am making a US$26 million film!’”

She says she cast Zellweger because the actress was “fantastic” in Nurse Betty (2000), a black comedy in which she plays a Kansas waitress; and “likeable and warm and grounded” as Tom Cruise’s love interest in Jerry Maguire (1996). Zellweger admits that taking on such a beloved character as Bridget was daunting.

“I had no idea if it was going to work at all – if I knew how to behave like her, if I was going to look like her, as I wanted to, as she’s described in the book.”

She agreed to spend six months prior to shooting in London, preparing for the role.

“I listened to The Archers, I watched EastEnders and Ali G on TV. I watched the news. I read magazines to see who were the current-day icons in English culture, to get an idea about who she might aspire to emulate,” she says. “After a while, I was humming along to Spice Girls songs.”

She also worked as an intern in a publishing house for three weeks.

Zellweger’s weight gain for the role (about 12kg) has been catalogued at length, and she can’t quite suppress her frustration when I raise the subject.

“I don’t think about it like weight gain,” she says. “She just needed to look like her lifestyle, and this is a girl who loves her ice cream. She drinks a lot of chardonnay, smokes a lot and goes to the bar not the gym. It’s like cutting her hair or wearing those black boots or the black bra with the see-through top – it’s about creating this character.”

There are scores of actors who might have worked as Mark, but Fielding was so taken with Firth’s portrayal of Mr Darcy (specifically the wet-shirt scene) in the BBC series Pride and Prejudice, broadcast in 1995, that she fashioned her brooding character after him.

“I thought it would be amusing,” says Firth, of playing Darcy the second. “I’d never done anything that had been referenced in popular culture. So when the columns were talking about Mr Darcy on television and I was being men­tioned by name, I thought it would be fun to get in on. I also had a feeling the film was going to do rather well.”

His favourite memory, he says, is of fighting with Grant in the Italian Garden fountain in Kensington Gardens for The Edge of Reason.

“Very, very funny and extreme and unchoreographed,” he says. “But oh my goodness, it was cold. It was November, it was about two degrees, and between takes you have to stay wet, so we had a little tub of warm water to sit in, in our suits, until it was time to get back into the cold water.”

For Bridget Jones’s Baby he had to carry a heavily preg­nant Bridget over Chelsea Bridge.

“No acting was required. It is my actual suffering you see on screen.”

THE COLUMN RAN WEEKLY from 1995 to 1998, first in British newspaper The Independent, then, from 1997, in The Daily Telegraph. After a gap of seven years, Fielding, by now living in Los Angeles and a mother (she has two children with Kevin Curran, co-executive producer and writer of The Simpsons; they separated in 2009) decided to revive Bridget, once again in The Independent.

The character in the new film is based on the Bridget of these 2005 columns, rather than Fielding’s most recent book, Mad About the Boy, published in 2013, in which Bridget is in her early 50s, has two children and is a widow following Mark’s shocking death. (“I did get a call from Helen saying, ‘Are you sitting down?’” Firth says.)

“The Bridget Jones’s Baby film was already in develop­ment when I had the idea for the Mad About the Boy novel,” Fielding explains. “And I just started writing instinctively, like writers do. It takes a lot longer to make a film than it does to write a novel – all you need for a novel is you, a laptop and a lot of chocolate.” She is unconcerned by the idea of two Bridgets at different stages.

“It’s fine. I think audiences are used to the idea of pre­quels and sequels and series of books and films coming out at different points in time. It’s a comedy series, not a history of the Battle of Trafalgar, and it wouldn’t be Bridget Jones to get everything in a perfectly straight line.”

Says Maguire, “I knew [making the new film] was going to be a slightly haunting experience – I was very close to the stories in the first one. But at the same time I was very nosy to know what had happened to the characters.”

In the years after making the first film, Zellweger was nominated for a Bafta and a Golden Globe for her work as Bridget Jones, won an Oscar for Cold Mountain (2003), and became so busy with back-to-back films that it threatened to overwhelm her.

“I was fatigued and wasn’t taking the time I needed to recover between projects, and it caught up with me,” she told Vogue recently.

She took a six-year break from Hollywood, yet agreed to return as Bridget.

“The Bridget experience seems separate somehow,” she says now. “There is a family involved. I share a long history of awkward moments with Colin and different kinds of struggles with Sharon and Eric. These relationships are grounded in something substantial.

“Bridget Jones is always there,” Zellweger continues. “Whenever I go out somebody brings her up: ‘Love Bridge Jones’; ‘I am Bridget Jones, I have to tell you what happened’; ‘No, no, that scene in the film – I am her’. It’s a blessing to be part of something that means so much to so many people.”

Bridget Jones’s Diary launched Firth into romantic comedies such as Love Actually (2003) and Mamma Mia! (2008). “I was now doing films where I was one of the guys on the poster,” he says. “That had never happened before.”

And now, of course, Mark has a new rival.

“It has a tremendous history,” Dempsey says, “and I didn’t want to know too much about that because that would have inhibited me. The character [of Bridget] is so beloved.”

Will there be a fourth film, I ask Fellner.

“I really hope so,” he replies. “The characters Helen has created are people you want to spend time with; you want to see what happens to them. It’s like being with your mates.”

Maguire says, “I very much wanted to bring Bridget to a younger generation. She’s out there swinging in the wind on her own. The only people she hangs out with are the new 30-somethings – that was exciting for me.”

“I think,” says Fielding, “at heart Bridget is about the gap between how people feel they’re expected to be and how they actually are. And Bridget Jones’s Baby is about the gap between how people expect their life to turn out and how it actually does. Increasingly, we inhabit a world in which the external – beauty, fame, thinness – is celebrated more than being human, warm and kind.

“I think it’s especially hard for kids now, when they’re all judging themselves by how many “likes” they get online. It’s great that Bridget seems to have quite a following among teenage girls. I hope it helps them remember that being a good person and a good friend is more important than having a bottom like two snooker balls and a posh handbag.”

Telegraph Media Group Limited 2016

Bridget Jones’s Baby opens in Hong Kong cinemas on September 15