“SORRY, I’m banging on now.” This the last thing Alison Steadman says to me. “You’ve opened a door and you’re sorry you’ve done it, aren’t you?”

Just before you joined us Steadman had, as they say, gone off on one. For the previous five minutes she has been giving me a passionate, exasperated rant about political opportunism, the brilliance of David Attenborough and Chris Packham, the terrifying disappearance of the Amazon rainforest, her membership of Greenpeace, and the loss of habitats for tigers and elephants and koala bears.

It’s a disquisition that is one-part grumpy old woman – “I know people who don’t drink water unless it’s out of a plastic bottle. Get over it and drink some tap water” – to three parts righteous eco-warrior.

“This morning on the news a whale was washed up on a beach … In fact, it was in Scotland, Harris … it had tons of plastic in its stomach. Tons and tons. Fishing nets and plastic bags and plastic bottles. God help that creature. It didn’t just die, it suffered. It starved.”

“I haven’t used a plastic bag now for eight years and I will not ever use one. I don’t care. I’ll carry things stuffed up my jumper. But people are still using them. So many people are in their own worlds and totally selfish. We’ve got into bad habits and we’ve got to change those bad habits.”

Whilst Steadman is in full flow I’m mostly thinking, “Go on yourself, Alison. Fight the power.” But, also, somewhere at the back of my head I’m remembering Steadman’s bespectacled hippy in Mike Leigh’s classic TV drama Nuts in May and thinking: “This is all very Candice Marie, isn’t it? She’ll be telling me she hugs trees next.”

Turns out she does. “I love trees. There’s a tree on Hampstead Heath and me and my friends go every year and we hug it. It’s called a paper hanky tree. I think it’s Chinese. And for three weeks of the year the leaves look like paper hankies. We love this tree.”

It is now almost compulsory to include the phrase “national treasure” when you write about Steadman. Her first IMDb credit goes back as far as 1971. And in the four decades and counting that followed she has quite simply been one of TV and cinema’s most reliable pleasures, most notably for her roles in her ex-husband Mike Leigh’s TV plays and films, including the aforementioned Nuts in May and Life is Sweet.

Of course, for most of that time, she was best known as the Demis Roussos-loving, social-climbing monster Beverley in Leigh’s 1977 classic Abigail’s Party. But over the years she also appeared in The Singing Detective, Pride and Prejudice (the 1995 wet-shirted Colin Firth take on the Austen novel), and turned up as Adrian Mole’s mum Pauline in Adrian Mole: The Cappuccino Years.

All of this, however, was before Gavin & Stacey. These days Ruth Jones and James Corden’s sitcom is the reason Steadman is recognised in the street. Steadman plays the titular Gavin’s mum, Pam, one of the sitcom’s finest creations. All cleavage and Essex bonhomie, Pam Shipman loves her “little prince” Gavin (played by Matthew Horne) and her husband Mick (Larry Lamb). She also has a bit of a thing for Prince Charles.

There is a new Gavin and Stacey Christmas special this festive season. You might have heard. People clearly have. “Every time I go out, I get stopped by various people going: ‘Oh, I can’t wait,’” Steadman confirms.

“It’s quite scary really. I’m thinking, ‘Oh God, the more people build things up …”

But she thinks we will like it. “I think it’s a very good episode. It is very exciting to be part of something that is so popular, and we’ve got this amazing slot on Christmas night.”

Well yes. My daughters can’t wait, I tell her. Neither can I, if I’m honest. Will she be watching herself? “I probably will. I’ll be with my son and he said to me, ‘Mum, there’s no way we’re not going to watch it together.’”

To be honest, it’s only fair to point out that Gavin and Stacey can be a bit Marmitey. There are some people who can’t be doing with it. There are some people who, in particular, don’t like James Corden (because he’s successful or too loud or something.)

I’ve always loved it. It’s partly personal (it was the first thing that made my wife laugh after she had been diagnosed with cancer. No small thing). And it’s partly because, unlike so much television, it refuses to frame working-class life as problematic or monocultural. It celebrates it in all its variety.

I mean, Alison, it’s perfectly possible, isn’t it, that Pam and Mick, who live in semi-detached splendour in Billericay after all, may even be Tory voters?

“Oh God, I hope not. No, I think in the end she isn’t. I think she might go around saying, ‘I might vote Green this time, Mick.’”

Well, is Pam the kind of woman who would shave a few years off her age, Alison? “Do you know, I’ve never thought about that. As anyone gets older you don’t want to keep hammering on about the fact that you’re 70-whatever, because it does seem quite old to young people. You’re half a step from the grave sort of thing. I don’t know. I don’t think she would. But I think it’s a subject she wouldn’t want to talk about.”

Steadman herself is 73 now and sanguine enough about it. Born and brought up in Liverpool, she’s now resident in London, where she lives with her partner, the Welsh actor Michael Elwyn. They stay five minutes from the tube and five minutes from the nearest wood (handy for any tree hugging requirements). She also likes a spot of birdwatching when she gets the chance.

It’s early December when we speak. She’s just finished four months filming in Manchester, working on Mike (Doctor Foster) Bartlett’s new TV drama Life and there’s a radio drama to be recorded in January. But otherwise her month will be taken up with a few Christmas readings and carol services for various charities.

“And I’m heading up the Marie Curie Daffodil campaign again this year. It’s a charity very close to my heart.”

And then there’s Christmas night with her son (her other son and her grandson is with the in-laws this year), watching Gavin & Stacey.

She sounds as surprised and delighted about the fact that the sitcom has returned as most of the rest of us. It’s been nearly a decade since the last episode was broadcast after all. Since then Corden has gone on to American late-night TV success and Jones has become a novelist. It looked to be destined to be banished to repeat status on Gold.

It’s fair to say she wasn’t expecting it. “I got a message from Ruth Jones just saying, ‘Would you be willing to take part in a Christmas special?’ I nearly dropped my phone when I read it.”

The first read-through was, she says, emotional. “Everyone was really tearful and hugging and saying, ‘I can’t believe this is really happening.’ It was like a family getting together again.”

That’s why she thinks it’s so popular, actually. It’s all about family; the one you’re born into and the one you create, whether that be in Billericay or on Barry Island. “You feel the love between them all, between the families, and I really like that,” Steadman says.

Her own family life started in Liverpool. “It will always in my heart, be my home,” she says. Can you still do your Liverpudlian accent, Alison? “I don’t know what you’re talking about. Don’t be so rude. What do you mean?” she replies, black affronted, while proving that he can sound proper Scouse when she needs to.

Born in 1946, Steadman was the youngest of three sisters. Inevitably, given the time of year, we talk about her earliest Christmases. She paints a picture wreathed in nostalgia and love.

“My mum was a brilliant cook and in those days, nobody bought mince pies, nobody bought Christmas puddings. She would make everything. Christmas cake, mince pies. She’d work her socks off. And I can remember my dad coming home with the turkey on Christmas Eve and setting about plucking it. Can you imagine that now?

“One of my strongest memories is going to see Father Christmas in a department store in Liverpool. I think I was four, maybe just coming up five. I’ve still got the photograph. My mum took me, and I can remember I was so excited.

“It was in the days when you could sit on Father Christmas’s knee and have your photograph taken without any problems. And I remember getting hold of his beard and looking underneath to see it if was real, because I didn’t believe it. Not in a horrible way. Just checking.

“And my mum saying, ‘Oh, don’t do that to Father Christmas.’ And me thinking, ‘Why not? I’m just checking if his beard is real.’”

She still has a picture of that day. “Ooh, it just warms my heart when I see it because I’ve got my best clothes on. I’ve got this little felt hat that a neighbour made me. She used to make hats for kids. So, I’ve got my best hat on and I’ve got my gloves on, my woolly gloves, and I’ve got the present that he gave me wrapped up. It was a stencil set, and I was very excited about that.”

Steadman tells me about another photograph, one of her mum and dad, George and Marjorie. “They were very kind, loving parents and I’ll always be grateful to them. I’m looking at a picture of them right now actually. I’ve got a picture of the two of them framed on my living room mantelpiece. They were so important to me, both of them.”

Her dad worked for an electronics firm Plessey. Money was always tight, but they were always encouraging about her chosen profession. To her face anyway.

Not long before she died, her mum told Steadman that she was really worried that her daughter would leave her family behind when she went to drama school.

“She apparently said to my father as the train pulled away … And it makes me want to cry every time I think about this … She turned to my father and she said, ‘Oh dad, I hope she doesn’t grow away from us.’

“And I never did. I went back all the time. I loved my family. I loved my parents. They were great they were so supportive.”

Steadman says she wanted to be an actor since the age of nine. After drama school she found her feet in the profession almost straight away. In 1973 she married Mike Leigh. They were married for more than 20 years and worked together on the roles that made her famous, most notably Abigail’s Party.

But I want to talk to her about the part that made her infamous. In 1986 Dennis Potter’s The Singing Detective proved a sensation when it was broadcast on the BBC. Part of that was down to the fact that it was brilliant, part of it down to the fact that it horrified middle England. It contained an adulterous sex scene between Patrick Malahide and Steadman (overseen by her character’s 11-year-old son). It inevitably prompted Daily Mail headlines and complaints from Mary Whitehouse.

I wonder what it was like to live through that media storm? Why did it cause such a fuss? “Well, it was because there was a child watching his mother having sex with a stranger,” Steadman accepts. “I mean, alright, when you put it in plain terms like that it is quite a worrying thing. But Mary Whitehouse, one or two others, they were all sounding off. And, of course, the Daily Mail leapt on it and just wanted to make a meal of it like some newspapers do.

“It was horrible to go through. It was absolutely awful, particularly for my parents. In fact, some newspaper managed to get my mum’s landline and rang her up. Luckily, she was sensible enough to put the phone down. I literally had to go to ground and didn’t speak to anybody.

“My parents lived in the suburbs of Liverpool. You can imagine the neighbours. “Of course, when we filmed that scene the boy wasn’t in the tree. And, actually, he wasn’t 11. He was much older. He looked that age, but he certainly wasn’t around when we were doing that scene. But they don’t know that, and they don’t care about that.”

How long ago that was. In the years since Steadman has moved from tabloid target to much-loved status, somewhere just to the left of Dame Judi Dench in the nation’s affections.

She knows how fortunate she has been. “Listen, I feel incredibly lucky. I’m 73. I’m still working. I’m still enjoying acting. I’ve got two lovely sons. I’ve got a little grandson. The sun is shining today, it’s a glorious day. I’m touching wood when I say I’ve got my health because without that you’ve got nothing.

“So many people have departed this earth recently that I’ve known which makes me very sad. I appreciate every day, I really do, because I think the older you get you realise you must value every day and not waste time.

“I try to keep as busy as possible. I love keeping busy, yeah, and if I can fit three different things in a day I will. As long as you’ve got the energy, do it … Because there will be a day when I won’t have the energy and then I can sit at home in the armchair.”

Gavin & Stacey will be on BBC1 on Christmas Day at 8.30pm