"Don't forget to put your blue plastic bags on.” It is another day's filming on the Gavin & Stacey set and everyone apart from the cast has to don protective wrapping for their feet. Outside it is muddy December in South Wales, inside it is supposed to be the swish, shag-piled Essex des res of Stacey's parents Pam (Alison Steadman) and Mick (Larry Lamb). So it is plastic bag time.

Everyone seems so relaxed you would not know that a couple of days ago this cute, cult BBC Three sitcom had bagged a brace of gongs at the British Comedy Awards. The story might have been mainly about the whirlwind Billericay-meets-Barry Island romance of Gavin and Stacey (Mathew Horne and Joanna Page), but the talk now is all about the whirlwind success of James Corden and Ruth Jones, who created G&S and play Smithy and Nessa.

As the young lovers sailed off into wedded bliss, the first series ended with a cliffhanger for the supporting duo. Brassy Nessa was pregnant after a lust-based encounter with happy-go-lucky handyman Smithy. Will she keep the baby? Will they wed? And, most importantly, can the sequel match the brilliance of the beautifully observed debut?

Between takes we chat in Ruth Jones's trailer, in front of a pile of Nessa's frilly underwear. With expectations high, the lively, likeable Corden, who made his name in The History Boys, is well aware that the honeymoon might be over if they can't keep up standards. “We felt the pressure even before the awards. People really seem to love it. And every time someone comes up to me and says they love it I'm thinking, ‘I hope we aren't the kind of show that lets people down.'”

For anyone who missed it, Gavin & Stacey is both a welcome throwback to the pre-Gervais days of accessible comedies with wide appeal and a knowing no-laughter-track critique of modern sexual manners. At its heart is the gently compelling love story between the two titular protagonists, cleverly complemented by the broader humour of English and Welsh family and friends orbiting around them, forever shuttling up and down the M4. Rob Brydon's meek Uncle Bryn, terrified of crossing the Severn Bridge, is a particular delight, while Nessa's PVC mini-skirts and boozy, brusque way with men lend the show an earthy feel. Think Wendy Craig's Butterflies with more lager, Judi Dench's A Fine Romance with more thongs. NBC has just bought the rights to develop an American version.

Ruth Jones, the slightly more serious half of the creative team and best known as barmaid Myfanwy in Little Britain's Only Gay in the Village sketch, tidies away a bra as she reflects on the success of a small comedy that punched way above its digital channel weight: “The second series was commissioned before the first one went out. It was the first time BBC Three had done that.”

As with series one, series two has an unfolding, filmic plot, something Corden, 29, is keen on. “I like the fact that you can watch the whole thing in one go. Mike Leigh is a big influence. I appeared in his film All or Nothing and learnt lots. I love the way he uses a narrative arc to get at truth.” Smithy and Nessa are in the foreground just as much as the newly- weds this time round, getting to grips with their on-off relationship: “It very much feels there are two stories about these four characters who go on a rollercoaster,” Corden says.

Jones, 41, adds that believability is vital. “We wanted a comedy that had a mainstream feel, but not bland ...” Corden adds “... a show that didn't exclude anyone. So many comedies either exclude me or my parents. We wanted to make a show that was first and foremost truthful. It feels like sometimes people aren't bothered about telling stories any more.”

Success brings further pressures, such as finding the time to write together. Jones lives in Cardiff, Corden lives in London and they are both increasingly in demand. Jones was in Steve Coogan's Saxondale and is in a TV version of Tess of the D'Urbervilles, Corden is writing a sketch show with co-star Mathew Horne, working title Corden and Horne Have Come. But getting together pays off. Gavin & Stacey has a sublime deftness of touch. “It's all about finding those tiny moments,” says Corden. “If this was an American sitcom almost every character in it would be the character that pops in and delivers the telly where the main character lives. These are the little quirky people we want to follow.” On a good day the writing just flows, Jones adds. “Sometimes it feels like they are not our characters, we just get to play with them.”

Three months on from the location visit I catch up with Corden on the phone while he is driving to the final dubbing session. He has had a busy week, presenting the NME Awards and some theatre awards where he snogged his hero Daniel Radcliffe (“I asked him first, I didn't pounce. and there were no tongues”). Well, he has collected so many awards it is about time he gave some out. He is loving every minute of it. Except when I ask him once again how he is coping with the pressure. There is such a long pause I wonder if he has tossed the phone out the window.

“I do have times when I get anxious. But all I have to do is watch it and it is plainly good. People describe it as a sitcom but I would never refer to it like that. I think it's ultimately just a story that's funny.”

The biggest pressure of all will be to stop at the right time. Inevitably the BBC wants a third series but having created a thing of beauty they don't want to milk it to death. “We are doing a Christmas special, but Ruth and I won't write another series unless we feel we can write a better one. We owe it to ourselves to know when to stop.”

Gavin & Stacey, BBC Three, Sun, 9pm

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