Happy Valley ten years on: Sally Wainwright reveals the key ingredients that made the Calderdale-based TV series such a huge success

It feels like the telvision equivalent of Adele doubting the popularity of a best-selling album.

But Sally needn't have worried. Happy Valley has engendered a love and a following from its audience that most screenwriters spend their whole career striving for.

"I put the same amount of passion, love and care into everything I write," Sally says.

"You want it to be understood, you hope that people watch them and get them and aren't thinking 'well I don't really know what that was about'.

"We were worried about that when we were making Happy Valley, that people would just not get it or it wouldn't resonate with people.

"You never really know whether you've got a hit on your hands until it happens.

"You put all that care into it to make the best show possible and you never really know until it happens."

Happy Valley had all the ingredients to become a smash hit; a gripping story, a very talented cast and some fantastic characters that were brilliantly written by a creator at the top of her game.

"I think people really like Sarah (Lancashire), they love watching Sarah," says Sally.

"I think that character was very likeable, even though she's quite challenging. She's a very likeable character and I think Sarah makes her likeable and very engaging.

"I think people liked her domestic set-up, they liked seeing her and Claire, sitting in the back yard together, talking.

"I think they also liked Tommy, I think they found him a very likeable villain.

"And the premise of the show, this conflict between Tommy and Catherine, I think it was the story.

"With a show like that, it's a number of things that make people excited about it.

"It's been phenomenal, it's been the most successful thing I've written by quite a long way.

"It's just a ripping yarn, it's just a really, really good story."

And at the heart of it is Catherine Cawood, played so magnificently by Sarah Lancashire, a character Sally feels encapsulates what Happy Valley is about.

"It would be Catherine with a sardonic aside, when she says 'I'll leave that with you, t**t'.

"This is why I'm writing Riot Women - I feel as a woman that you tell men things and they don't listen and later on you say 'well, I did tell you'."

Let's not forget Calderdale's role in the show too.

"It's the landscape I think, it's become a very important part of the show," Sally says.

"It feels very, very centered in that part of the show, a huge part of the atmosphere of the show is the landscape."

There can't have been a Calderdale based viewer who didn't indulge in some location spotting at some point.

"It's about capturing some of the quirkiness of some of the buildings really," Sally says.

"There's a beautiful shot that Fergus (O'Brien, director) got in the last block of the last series where Tommy was staying when he escaped. There was a fabulous view across Halifax, I think it was the Shibden Valley.

"It's things like that, when you're choosing your location so it really captures the beauty of the place but also some of the quirky back streets."

Sally says she didn't initially envisage it being a three series show.

"When you start writing a new series, like Riot Women (Sally's latest project), I hope I'll get a second series, I hope it'll run and run, I hope people like it but you never know how it's going to go down, whether you're going to get a second series or a third series," she says.

"After series two, me and Sarah had a conversation about it and we decided we were going to do one more series and then that would be it.

"Sarah's very good at knowing when to stop and I've always admired that about her.

"We decided, because the show had been so successful, we wanted to be in control of how and when it ended rather than it going on and on and on, to control how it ended, who ended up winning in the contest between Tommy and Catherine and then to leave it in people's memories.

"Some very successful shows have only done short runs, and it was just a very conscious decision that me and Sarah made."

Does Sally have a favourite series of the three?

"They're all different and I like them all for different reasons," she says.

"They're all challenging for different reasons as well, it's always been a hard show to shoot has Happy Valley, it's always been a traumatic experience.

"Every series, things have gone wrong.

"My producer was at an awards ceremony the other day and she said 'Have you got any messages to send if you win?' and we joked we ought to tell people that if only they knew the pain and misery we went through!

"We loved making it as well, but every series we had to shoot in winter for a start, which doesn't help, and it's a tough shoot.

"In the winter the weather's bad, your daylight hours are limited.

"The second series, our production base was in Elland and luckily for us but not luckily for Elland, the day after we finished filming, it flooded.

"That was a bit of luck on our part but normally it was the other way round and everything was stacked against us, so it's quite nice and sweet that we've ended up winning some awards for it!"

Despite her love of the show, Sally hopes Happy Valley doesn't overshadow her other work.

"I hope not," she says when asked if she thinks Happy Valley is what she'll be remember for the most.

"I hope I've got loads of other stories to tell.

"It's possible, it's certainly had an impact, not just here but in the United States as well.

"I'd like to be remembered for Gentleman Jack as well, and Riot Women, and a few other things.

"But it's certainly the show that, so far, has had the most impact."