The late rise of Ruth Jones is a blessed relief. We all know the picture on television these days: what used to be a talent-based economy is in deep recession. According to the prevailing rules of ageism and lookism, Jones should still be plugging away in supporting roles, typically as the large, gobby sidekick - which, for years, looked like the outer limit of her casting range.

Last year, however, after 20 years in the business, and at the ripe age of 41, she won best comedy female newcomer at the British Comedy Awards.The performance that swayed the jury was still a gobby supporting role. This time, however, Jones - with James Corden (who won the male award) - had written it herself. Whether by accident or design, there were few laughs in Gavin & Stacey for either Gavin or Stacey, but all sorts for their sidekicks, Smithy and Nessa. “We didn’t do it intentionally,” she insists. “Everyone says, ‘Oh, yeah, they obviously gave themselves all the best lines.’ We literally start improvising.”

After two series, the show will have possibly its last outing when the two families from Billericay and Barry, united by the titular marriage, come together for Christmas just as the entertaining Gavin & Stacey annual finds its way into the seasonal stockings of true obsessives. It might seem rash for Jones to kill off the source of her own success, but she makes a convincing case for quitting while ahead. “We’re very, very keen for it not to become boring and recycled. It’s not out of the question that we would write a third series, but the way things are for me and James, we haven’t got the time to think about writing it.”

She has a number of other irons in the fire. Having popped up as the heroine’s mother in Tess of the D’Urbervilles, she is currently back in the 19th century as poor old Flora Finching in Little Dorrit. Meanwhile, every weekend in Cardiff, she is hosting her own Sunday-morning show on BBC Radio Wales. And there is a new career as a scriptwriter, with two projects on the go.

For quite a while, the nation mostly knew Jones as Myfanwy, the sapphic pint-puller of Llandewi Breffi, the un-Welsh-looking village in Little Britain that has but one gay. From behind the bar, she cheerfully exhorted the diffident Daffyd in a singsong Swansea accent: “You could have had a bit of cock there.”

“When I was sent the Little Britain sketches, I remember reading it out to my husband and going, ‘This is really rude’, but smiling when I said it. There’s something about the intention behind it.” But although she joined the show’s theatre tour, Myfanwy was no more than a very public cameo.

When I first met Jones, on the set of Saxondale, she mentioned something about a sitcom of her own brewing. She was in make-up, having a tattoo printed on her cleavage to play the girlfriend of Steve Coogan’s roadie-turned-pest controller. Coogan offered her the part after she played a dunderheaded beautician from the Rhondda Valley in Nighty Night for his production company, Baby Cow. The cheerful, patient Magz was the snuggest fit of anyone she has played. “The character in the first series was very much like me in real life.” Certainly, the bright smile in that open, attractive Welsh face seems entirely natural. Unfortunately, by the second series, Magz had been written into the wallpaper, “which was a shame. I would have liked to have done more. I’d make snide remarks to Steve Coogan”.

She would have minded her downgrading far more with no Gavin & Stacey up her sleeve. Jones was the driving force who made Corden sit down and write up their idea together. The series went out on BBC3 and might have remained in that ghetto, but something about its big, beating heart propelled it to wider popularity. Jones’s character, Nessa, was the most alternative thing in it. An outsize goth with a taste for leathered adventure, she works the Barry Island slots and reminisces about a fantastical past involving, among others, John Prescott, Nigel Havers and a couple of Fayeds. In her seen-it-all Cardiff accent, bog-standard Welsh lexicon was winningly reminted as a set of catch phrases: “Tidy”, “Genuine”, “I not gonna lie to you”, “O, what’s occurrin’?”. Jones defends her to the hilt. “She’s got a real soft side to her, Nessa,” she insists. “She is incredibly accepting of people’s faults. She says, ‘I’ve been judged myself, both inside and out, and it’s not nice.’”

Jones started out in acting on the assumption that she, too, would be judged. She grew up in Porthcawl, where her mother was a doctor and her father worked as a legal executive for British Steel in Port Talbot. “We had seven beaches. I always felt really safe.” That sense of security didn’t entirely apply to her acting. Though the star of sundry school shows (alongside Rob Brydon), she didn’t apply for drama college. “Everybody else did, but I was just thinking, ‘Oh, God, I’ll never get in.’ I suppose the thought of being that competitive was too much for me. All my life I’ve had this belief that other people are better than me. It’s only now that I’m starting to realise I’m all right, that I’m not bad at what I do, actually. It’s a shame it’s taken me until 42 to realise that. I wish I’d thought of it a bit earlier, because it wasted a lot of time.” Having read drama at Warwick, she refused to to look at the job listings for TV on the grounds that “I genuinely thought I could never be on television”.

After her first job, she moved to London and worked in the education department at Kensington and Chelsea borough council. There was “quite a long period where I was thinking, ‘I’m going to give this up’ ”. But she wasn’t entirely without courage. Roger Michell’s 1995 production of Under Milk Wood at the National Theatre was a job-creation scheme for Welsh actors. Though she didn’t have an agent, Jones faxed the casting director. “I just said, ‘I’m big, buxom and Welsh, and you should see me for this production.’ Completely not expecting anything. I had to negotiate my own contract, my voice squeaking on the phone. But I did actually get my money up.”

The screen part that quietly ignited her career was in East Is East. Her gobby northern lass got her cast in Fat Friends. For four series, she played an overweight blonde from Leeds. It was here she clicked with the much younger Corden. In Little Dorrit, she is playing another sort of fat friend. Flora Finching was Dickens’s cruel portrayal of his young sweetheart Maria Beadnell, whom he met again in middle age.“Flora, always tall, had grown to be very broad too, but that was not much. .. Flora, who had been spoilt and artless long ago, was determined to be spoilt and artless now. That was a fatal blow.” In the 1988 film version, she was played by Miriam Margolyes. Jones messed up her first audition and heard nothing. Even a couple of years ago, she might have done nothing about it. But the best female newcomer of 2007 is made of stronger stuff.

“I saw the executive producer a little while later and said, ‘Why didn’t I get that part? It had my name written all over it.’ They said, ‘We haven’t cast it yet. Maybe you could get reseen for it.’ I don’t know if this is because of Gavin & Stacey or my age, but I believe in myself more. I don’t have any qualms. I don’t think it’s arrogant. It’s just a practicality.”

Little Dorrit is currently showing on BBC1; Gavin & Stacey will be on BBC1 from Friday

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