Gavin & Stacey: An exclusive behind-the-scenes look at the Christmas finale

"It can't carry on after this," explains James Corden. "It just can't."

Standing outside his mobile dressing room in a car park on the outskirts of Cardiff, the co-creator of Gavin & Stacey is explaining why the sitcom is ending after 17 years.

"Look, obviously there's lots we can't talk about. But Christmas Day will be the last time that we ever see all these characters get together."

He insists that it is "not up for debate" and the show, which he also co-writes and stars in as Smithy, will not be, to use the show's distinctive parlance, occurring again.

"There are some things happening in this special which really, really tell us that has to be it. We can't see a way how it could go on. So that's the reason to end it now."

From 21:00 on Christmas Day around the country, this much-loved show will be wrapped up amongst the wrapping paper.

It is just after 8am on a grey October morning. BBC News has been invited to spend a day watching one of the final ever filming days of Gavin & Stacey for a half-hour iPlayer special.

Our first stop is the make-up truck, where Ruth Jones, Nessa in the show, is having a Welsh dragon tattoo applied to her arm.

"The girls are always in before the boys," she laughs.

"James Corden is in for five or six minutes at most. I'm in for an hour and a quarter. But the results speak for themselves," she chortles, "And he misses out on the really, really dirty, disgusting gossip."

One mirror along, Joanna Page is being turned into Stacey, although hearing her chat to the crew, she is pretty much Stacey to start with.

Her subjects for that morning's session include Christmas decorations, a change in the weather ("a great topic") and ear piercings ("at the end of filming I'm going to get another to mark that the whole thing is done").

James Corden arrives to use the beard clippers and perhaps inadvertently gives away one of the more telling little nuggets of detail about the show, when he explains his stubble must stay that length: "Because it's only set over four days."

Rob Brydon, already wearing Uncle Bryn's finest brown party clothes, is ruthlessly teased when he arrives. The cast are heading out for a team curry that evening and it has been decided that he should pay, having managed to host a corporate event and squeeze in a voiceover gig around yesterday's filming.

He pulls a that-won't-be-happening face, before saying his major concern is when the dinner will finish, due to the early starts on set.

Soon he has the whole room laughing with his impression of Ronnie Corbett having his make-up done.

They are clearly a cast who love spending time with one another, even this early in the morning.

Jones has headed back to her trailer. When I walk past five minutes later, she is standing in the doorway holding a plate, which she tilts towards me: "See we do eat omelettes on this show," she laughs, "Although this one was not made by Gwen."

After a short minibus ride to the commuter village of Dinas Powys, the cast arrive at a very familiar detached house, in totally the wrong part of the country.

For 17 years, this is where scenes at Pam and Mick's have been filmed, with the Vale of Glamorgan, doubling up for Essex.

Tents have been erected on the outside of the garage, to provide room for all the equipment and monitors.

We watch as a party scene involving limbo dancing, is filmed from every angle, as Bryn downs shots and Nessa gives Smithy a lesson in how to eat the Greek yogurt dish, tzatziki. ("Don't use the cracker. Use the bread.")

What is not clear, is how this follows on from the proposal cliff-hanger at the end of 2019 Christmas special, which was watched by 18.5 million people, more than a quarter of the UK population.

"Well, I can tell you is it isn't set at Christmas," shares James Corden, who explains having already set one Christmas Day special in Barry and one in Essex, it was felt that concept had been done.

"I can tell you it's set after that moment. There's no time jump, we're not going back in time," he adds.

Perched between monitors and directors' chairs, Jones explains one major change: "It's 90 minutes. And all of the characters have got their own little story in there, which is lovely. Because if you are fond of a show, you love all the characters.

"It's a bit like, I used to love the Wombles when I was little and I would love any story about any of the Wombles."

However, Corden refutes the idea that the length now means it is a film. "I don't want to say it's a film because I think that brings expectations of scale and that I'm not sure our show could ever reach."

Sitting on a sofa that has survived from the first series, actress Alison Steadman is loving being back in Pam's domain, but says she is "really, really dreading" the final day of filming.

"It's been such fun over the last 17 years," she says. "People love it and stop me all the time to talk about it. I love Pam. She's so crackers."

She too is confident that all is going to end well, describing the finale as having "storylines that'll go in a direction that people aren't anticipating."

At this point the house's actual musical doorbell rings out, and we both laugh at the very Pam-ness of it all.

Her onscreen husband Larry Lamb has spent much of the scene that is being filmed, standing behind Mick's bar pouring shots, while wearing shorts.

He believes the show's continued popularity is to do with the way: "Everybody in it represents an element of the life of contemporary Britain.

"Everybody's in there. And if they're not, somebody they know, and love is in there in that form."

For Mathew Horne, having played Gavin for 17 years is throwing up all sorts of existential questions: "I'm putting to one side all my neuroses about that and to the ageing process."

"It's incredible because it is part of people's upbringing, you know. And we were all in their homes as they were growing up, and now they're adults and out in the world, and it's extraordinary how much the show means to people. So yeah, it's a lifetime for me, but it's a lifetime for so many other people."

At the end of the street, a crowd of about 50 fans have gathered to watch the last week of filming, including Lisa Lacking who has travelled down from Cheshire.

Through the fan community she has become friends with Brenda Kenyon, who owns the house which is used on screen as Stacey's family home. And during this she is sleeping in the room used for Stacey's bedroom.

Her hopes for the last episode are simple: "There's got to be wedding."

Rob Brydon is adamant that fans will be satisfied: "It's a lovely ending to the show. There are definitely surprises. Things that we as the cast went, 'What!'"

He also reveals that he cried whilst reading the script to his wife: "She thinks the ending is terrific."

Back inside, the cast are becoming nostalgic. Corden points out the exact spot in the kitchen where Smithy made his curry order, before gesturing towards the stairs where Pam and Mick once appeared in matching kimonos, revealing that one crucial piece of the show will take place there.

With the 2019 Christmas special having been set in Wales, Joanna Page has not been in Pam and Mick's for 15 years, so is bamboozled by the lay out of the house ("I don't remember this mezzanine walkway") but is making the most of every day.

"Knowing that this finally is the last one, sort of makes you savour everything just all the more.

"These moments that we get together are so special because they're not going to happen again, in these circumstances, in these costumes, all of us together."

Of all the cast, she has made off with the most souvenirs, ranging from, "an ornamental cat type thing that might be a fish" from Stacey's house to almost the whole of the outfit she's wearing ("these boots are pretty fab, this is a haul").

Jones is hugging a cup of tea, waiting to be called for a scene where she will be dancing to Abba, and thinking about how close they are to the end, but how pleased they all are that it is on their terms.

"This is a full stop," she says ruefully.

"I think we are very lucky to get to choose to end it, rather than to be told, 'Sorry, we don't want anymore.' I think it's lovely to be able to say a very healthy farewell to it.

Before we go, I ask James Corden, how he would like the show to be remembered.

He takes his time to really think about this and then talks at length about what he believes to be the show's DNA, including "all the stuff that makes life good, which is friends, family and love".

The conclusion to his thesis is: "I hope it's remembered as a show that can make people feel good, that can bring comfort and a sense of warmth and history. That would be lovely if people spoke about it like that."

Now he is waiting to see how the finale is received.

"If we can land the plane safely and just give it over to everybody else on Christmas Day, I don't know, what a trip man, what an absolute trip it's all been."

Gavin & Stacey: The Making of the Finale, a 26-minute documentary, is available on the iPlayer now. The finale itself is broadcast at 21:00 GMT on Christmas Day on BBC One.