Its one-time cult fanbase has expanded massively in the past 12 months, thanks to rapid word-of-mouth praise and a highly acclaimed Christmas special last year, but that success inevitably brings with it incredible pressure to deliver the goods and to match the high quality of the previous series.

For the uninitiated, the show is about a sweet wide-boy from Essex named Gavin (played by Mathew Horne, who had a recurring role in The Catherine Tate Show) and sweet homebird Stacey (Joanna Page, best known as the porn star from the movie Love Actually), who lives in the tiny, and very eccentric, village of Barry, Wales.

In the very first episode of the first series, we learned that the pair had been conducting a love affair by phone and email, but had yet to meet in person. Stacey then travelled up to London with her best friend Nessa (Ruth Jones, who also co-writes the show) to meet Gavin, who brought along his own best pal, Smithy (James Corden, the other co-writer).

Gavin and Stacey fell in love instantly and eventually got married, while Smithy and Nessa conducted their own, erm, complicated non-relationship-relationship, resulting in the birth of their son, Neil, at the end of series two. The show also has a cast of wonderful supporting characters, including Gav's parents Mick (played by EastEnders villain Larry Lamb) and Pam (Alison Steadman), and Stacey's mum Gwen (Melanie Waters) and uncle Bryn (the scene-stealing Rob Brydon).

Gavin and Stacey makes no apology for being achingly sweet and romantic, but it has so far balanced out the treacle by also managing to be uproariously funny. Similar to Will and Grace, the majority of the big laughs in Gavin and Stacey come from the protagonists' best friends. Corden's Smith is a crude, immature man-child (but with a crucial heart of gold). His affection for Gav pushes the concept of 'bromance' to its limits by being uber-possessive, to the extent that Gav had to phone him every day during his honeymoon.

Ruth Jones' Nessa, on the other hand, is a chain-smoking, tattooed, leather-clad, Doc Martin-wearing truck driver-cum-amusement arcade manager, fluent in Italian and Welsh, and blessed with an ever-surprising and colourful back history that includes being a member of the original line-up of the band All Saints and a coaching goalkeeper for Coventry City, as well as conducting affairs with Richard Madeley, Dodi Al-Fayed, John Prescott and Russell Brand.

Gavin and Stacey first aired in May 2007 on BBC3, a channel that often serves as the testing ground for new series (Nighty Night, an earlier breakout BBC comedy hit, also debuted on the channel before moving over to BBC2). The show drew strong reviews from those who saw it and amassed something of a cult following. Throughout its first series, ratings averaged at around 560,000, but once the series began its repeats and came out on DVD, it grew in popularity through friend recommendations.

When the second series started, again on BBC3, in March 2008, viewing figures had climbed to 1.8 million. The show was now officially a hit. The special, stand-alone episode that aired on BBC1 on Christmas Eve last year drew 7.2 million viewers and was hailed as the best thing on television over the entire festive period by AA Gill, the acerbic TV critic of The Sunday Times. The show has also managed to bag two BAFTA awards and four British Comedy Awards in the past two years, while Rob Brydon and Ruth Jones recorded a version of Dolly Parton's Islands in the Stream in character as Bryn and Nessa, together with Tom Jones, for the 2009 Comic Relief.

All of this has raised the stakes considerably for the third series. James Corden admitted as much earlier this year: "People really seem to love it," he said. "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'."

There are two factors working against Gavin and Stacey this time round. The first is the backlash that has hit Corden himself over the past six months. Already in danger of being over-exposed, the 31-year-old was then panned for his hosting duties at the BAFTAs, then drubbed even more for he and co-star Mathew Horne's sketch show, Horne & Corden, which was dismissed as "excruciating and puerile" by one critic and as being "about as funny as credit default swaps". On top of all this bile, Corden and Horne then released their self-penned horror comedy Lesbian Vampire Killers to even greater levels of scorn.

Apart from Corden's confidence taking a knock, it also meant he didn't have as much free time to spend with Ruth Jones (42) writing the third series of Gavin and Stacey. For one thing, Jones lives in Cardiff, while Corden lives in London. Jones herself has been in great demand as an actress, starring in Steve Coogan's Saxondale and the TV version of Tess of the D'Urbervilles. The pair have said, however, that they still work as well together as they did on the first two series. Says Jones: "Sometimes it feels like they are not our characters, we just get to play with them."

The second big factor they have to contend with is what could be described as the 'Fawlty Towers Curse'. That comedy's writers and stars, John Cleese and Connie Booth, ended the iconic show after two series and refused to ever do a third one.

Unwittingly, Cleese and Booth invented something of a template for all British sitcoms since. The makers of such hits as The Young Ones, Nighty Night and Spaced have all stated that they followed the Fawlty Towers example of going out on a high after two series. Ricky Gervais also bowed out after two series (and a Christmas special) apiece on The Office and Extras. Therefore, Corden and Jones are bucking history somewhat in their attempts to continue the Gavin and Stacey magic.

Corden himself is trying to ignore all the high expectations. "I do have times when I get anxious," he has admitted. "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 [Gavin and Stacey] like that. I think it's ultimately just a story that's funny."

As for whether they will feel the need to extend the show even further into a fourth series, Corden says that it looks unlikely, but, as ever with television, nothing has been ruled out.

"Ruth and I won't write another series unless we feel we can write a better one," he told a British newspaper last year. "We owe it to ourselves to know when to stop."

W

Gavin and Stacey, BBC1, Thursday, 9pm