There’s nothing cool about Miranda Hart, and that’s precisely what’s special about her. In roles in sitcoms such as Not Going Out and Hyperdrive, and in cameos on everything from Absolutely Fabulous to The Vicar of Dibley, she’s plied the same persona: big, posh, slightly daft, yet amusingly at home with her own awkwardness. Lee Mack, who created the role of Barbara the cleaner for her in Not Going Out, says: “Miranda is naturally funny — BBC Two seems to allow itself one funny woman at a time, and now that Catherine Tate has finished her show, that job is hers.” The BBC appears to agree. Her sitcom, Miranda, is one of the key shows in its new season.

Hart is refreshingly uncynical about her tilt at the big time. “This is what I’ve been working towards all my life,” she enthuses, sitting on a sofa in an office in Television Centre. “It’s a dream come true. My earliest memory is wanting to be a comedian and to have my own show on the BBC.”

Miranda, it must be said, is as cheerfully uncool as its writer and star. Like the more gag-happy Not Going Out, it’s an old-school sitcom in a brightly lit studio with outsized characters doing outsized things. Hart plays — you guessed it — Miranda, a daffy, 6ft 1in thirtysomething who’s useless with men. She runs a joke shop with her short and fussy best friend (Sarah Hadland). She meets her awful friends from school (Sally Phillips and Katy Wix). And she’s harangued by her imperious mum (Patricia Hodge) for not having a husband.

It’s good fun, if you buy into it. And if you do, it’s because of Hart. She’s not just in every scene, she also confides with us straight to camera. She is, indeed, a natural. She’s wanted to be Joyce Grenfell since she was 18. She’s loved Morecambe and Wise for even longer. She could never see herself trying to make her own Office, her own Nighty Night. “It was a definite choice not to do that, to try and be big and silly and not be ashamed about falling over.”

The shows even end with the cast waving goodbye, one by one, preceded by the caption “You have been watching . . .” It’s a nod to Croft and Perry sitcoms such as Dad’s Army. “I’m saying this is what I’m doing and I’m not scared to do it. Some of my comedy peers do slightly fear being a mainstream figure, as if it’s slightly uncool. Well, I thought I’m going to embrace it.”

If there is a subversive element, it’s in the way she’s written in a would-be boyfriend character (Tom Ellis) who is just there as eye-candy for Hart to humiliate herself in front of. Hart insists that his character grows as the series develops. “But as I wrote it, I did think that this was quite satisfying, that I’ve done to a man what men have done to women for 40 years of sitcom!”

It’s the fate of most female comics to be compared to Catherine Tate or French and Saunders. Well, we’ve touched on Tate, so let’s move on to French and Saunders — after all, it’s because of them that the show happened. Miranda first came about when Abigail Wilson, who works for them, came to see Hart perform a solo show in 2003. She suggested that Hart pitch a show to the BBC. And, after a year or so of figuring out the format, Hart did a read-through of her script for BBC executives. Saunders was there, laughing throughout.

“People were crying with laughter at her crying with laughter. You could see commissioners thinking: ‘Well, she’s laughing.’ So we got lucky.”

That was five years ago. Since then she’s turned the television scripts into a radio series, and now back again. Overnight success really doesn’t happen overnight, does it? “It really doesn’t. But I never rushed this, because you just don’t get opportunities like this. I wanted it to be as good as possible. I added almost a year to the process myself because I had work, and I didn’t want to be doing Not Going Out and then coming home to write the script at night. I just wasn’t going to do that.”

Hart comes across as a less strenuous, stronger version of her comedy persona. “She knows what she wants,” Mack says. “She plays an extension of herself. She’s a true comedian, putting herself into the part, rather than an actor, changing to fit it.”

So how much of Miranda is there in Miranda? She grew up in Petersfield, Hampshire. Her mother has “never once” pushed her to get married — but Hart has snaffled some of her lingo. “I definitely had my ear to the ground when my mother had her friends round: ‘Oh, we must have a quiche! Such fun!’ I love that language.”

Like her screen self, Hart went to boarding school — Downe House in Berkshire. She was sent there at 11 when her father, who was in the Navy, was posted to America. She had a ball: “They were probably the best years of my life. I was the clown, I felt I could be myself, more than I’ve ever been until now.”

In the show, Miranda’s old schoolfriends call her Queen Kong in dubious tribute to her size. She has been 6ft 1in since a sudden growth spurt at 15. “Friends and family were very sweet about it,” she says, “but actually ended up creating a slight problem. They would go: ‘Models are tall! Embrace your height!’ They were worried that I was worried. But the trouble was I kept being told that I was tall. It still is, age 37, slightly odd being considerably taller than men. It doesn’t feel right.

“I used to be thin, and I was getting used to being tall. And then in my mid-twenties I ballooned in size. Then I was tall and big, and that I found difficult. That’s when I really started writing comedy.”

Hart put on weight when she was on antidepressants for a couple of years. She was uncomfortable with herself, she says. “And a sort of anxiety runs in the family. I remember being annoyed it was happening to me in my twenties. But now I think, much better that than in your forties.”

For a while, she tried to take the plums out of her mouth. Working as a temp in London to pay the bills, she used to muddy her accent. “I used to fear that people would think I’ve not had a problem in my life, that people would think: ‘Oh, Daddy’s bankrolling her.’ And that wasn’t the case.

“But I have had a few conversations with people going, ‘Oh, I can’t bear Jodie Kidd or Tara Palmer-Tomkinson, they’re just spoilt’, and then I find myself defending public-school people and going: ‘They can have suffered.’ It doesn’t matter if you were brought up in a castle, you can still have tough times.” She catches herself. “I wasn’t brought up in a castle, though!”

She studied politics at Bristol Polytechnic, but she always had her sights on comedy. Yet it took her until her mid-twenties to start acting. She formed a double act, the Orange Girls, with Charity Trimm — “that’s her real name”. Then, in 2002, she did a solo show in Edinburgh. She got an audience but lost money. In fact, it wasn’t until this year that she finished paying back the £7,000 a friend lent her for the show. And she only stopped temping in 2005, when she got the job on Hyperdrive.

For someone so self-deprecating, she certainly hung on in there. Was there a point at which she thought, yeah, I can do this? “There must always have been a sense of ‘I’ve got enough to carry on’. You know, however much I’m terrified and think I look and sound ridiculous, there is a confidence that keeps me going. Every comedian has to be like that. You wouldn’t get on stage if you didn’t think that you were good.”

Hart would like to act in some plays — “I’d love to do farce, I’d bring back Ray Cooney”. But at the moment she’s taking a month off. Trying to see some of the friends she’s neglected while spending all her time in either Television Centre or her flat in Hammersmith. Still, it had to be done.

“I spoke to some older comedians and they all said there was a period when they had to go into their tunnel, because work suddenly comes in. And then too much work comes in, but you have to take it. So in the last couple of years I’ve acted in three series, written two radio series and one telly series. I’d like to blame the fact that I’m single on that — it’s just that, the offers have come streaming through . . .”

Hart has taken her poshness, her awkwardness, her height, and found somewhere safe to put them. “Now I’m socially competent, but in my twenties I was as scared as that sitcom character. In a comedy way I can be big, I can embrace my height, I can be posh, I can fall over, make a tit of myself socially, and it’s all fine.”

So her sitcom has finally helped her to get the hang of herself. Well, almost. “I did go to the loo just now and walk into the men’s by mistake. So I am still a bit of an idiot. But then I’m hoping, aren’t we all?”

Miranda is on BBC Two on Mondays at 8.30pm from Nov 9

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