At the Bafta TV awards next Sunday evening, all eyes will be on the multi-nominated, high-octane dramas Killing Eve and Bodyguard. The comedy category, however, is dominated by a very different show: Stefan Golaszewski’s quiet, tender-hearted sitcom Mum, which has been nominated for four awards.

Starring Lesley Manville as Cathy, a woman taken for granted by all around her, and Peter Mullan as Michael, her late husband’s best friend who yearns to be with her, it is that rare thing, a word-of-mouth hit that has seen its audience grow with each series: the third and final one is set to begin on BBC Two on 15 May.

“I think one of the reasons people have responded to the show is that there’s a lot of negativity and scorn out there,” said Golaszewski, who also wrote the acclaimed BBC Three comedy Him & Her. “People are sometimes scared of emotion and it’s also quite hard to do. The journey in Mum is one of empathy, about being comfortable in your own skin. This is a show where there are no baddies, simply people who are misunderstood or in pain.”

Facebook Twitter Pinterest Adrian Dunbar as Ted Hastings in Line of Duty. Photograph: Peter Marley/BBC/World Productions

He also sees it as a response to the police procedurals and twist-fuelled dramas that dominate modern TV. On Sunday, nearly 8 million people are expected to settle down to watch the last episode in the fifth series of Jed Mercurio’s Line of Duty, which is now the most-viewed show of the year.

“I just felt so bored of plot twists,” said Golaszewski. “Aren’t we all a bit bored? I’ve seen so many of them as a viewer, so I feel as though I’m really educated in how they work.”

By contrast Mum is a show about littler lives. “I wrote it about the world that I’m from, which is lower middle class, with its roots in working class, because I feel that so much on telly is posh. There are so many shows where the people live in posh houses and they’re meant to be in London and you just think: how does this person live in this house?”

The casting was also crucial. “The most important thing was that while it’s a sitcom and so must make the audience laugh, it should also have an honesty at its heart. Lesley and Peter in particular, but all of the rest of the cast as well, understand that. They always play the emotional truth of every scene and they always make it look easy.”

He admits, however, that people initially misunderstood his intentions. “I did set myself a problem by introducing a load of sitcom types,” – in addition to Cathy and Michael, the show features scornful snob Pauline, her hapless partner (and Cathy’s brother) Derek, Cathy’s dim son Jason and his ditzy girlfriend Kelly, as well as Reg and Maureen, Cathy’s irascible in-laws – “but I knew that over three series I was going to chip away at them and get into why they behaved in these ways.”

For the show’s devoted audience the key question is will Cathy and Michael, whose slow-burning relationship lies at Mum’s heart, end up together? Golaszewski gave nothing away but said: “I would never cheat the audience. The way I picture it is as a person who comes in after a hard day at work and wants to feel something. That’s why they put the telly on and that’s what we need to provide.”

Some dismiss that attitude as “fan service” but he has no time for them. “You have to engage with your audience. You get people who say I’m just about my work and my art and I don’t care about my audience. I think it’s arrogant and rude. You should respect your audience and engage with them, and that doesn’t mean a race to the bottom. It means recognising that the people watching at home are humans with brains and hearts and lives.”

The new series takes a risk by moving the characters out of Cathy’s semi into the big house of Pauline’s dreams, compressing the timeline from the usual year to a week’s holiday, a decision that Golaszewski says has helped keep it fresh: “One of the nice things about mixing it up is that now everyone can talk to each other and you find new combinations and new relationships to explore. The last thing I’d want is for the audience to find it stale or repetitive. I don’t want anyone to think we’re just telling this story for the sake of it.”

Facebook Twitter Pinterest Writer of Mum Stefan Golaszewski, centre, with Peter Mullan (Michael) and Lesley Manville (Cathy). Photograph: James Veysey/Rex/Shutterstock

He always conceived of the show as a three-series arc. “I knew the journeys the characters were going on, where they’d end up and what that would ultimately mean for them. It’s such a specific story about grief and its after-effects, and a woman who has a crisis of identity and finds her next way of how to be. Once that story is over, if you carry on, then you’re just doing it for the money, which really isn’t a good enough reason.”

With that in mind, how does he hope the final episode will be perceived? He laughs and then looks a little worried. “Honestly I hope people are satisfied by the ending. It would break my heart if they weren’t.”