CHRISTMAS Day isn't complete these days without Downton Abbey on the box, and viewers will no doubt be warmed to hear that, unlike last year, the upcoming special will be set during the Christmas period.

"I think audiences will really enjoy this one. It has a lovely festive element to it," says Hugh Bonneville, who plays the show's patriarch Lord Grantham. He's keen not to divulge too much, but will reveal that "a chunk of" scenes have been shot at Alnwick Castle.

"Highclere [Castle, where Downton is usually shot] is stunning to film in but frankly, Alnwick Castle is huge. That was the highlight of the year," says the London-born 51-year-old.

"Harry Potter filmed exteriors there, but I believe it's the first time cameras have been allowed inside. The Duke and Duchess of Northumberland are huge fans, which is why they let us in."

And they're not the only ones. Downton's become something of a phenomenon since it first aired in 2010.

"We knew something odd was going on when, after the launch of the show, when there's usually a bit of a dip, the viewing figures went up. I'd never known that to happen and it just grew and grew because the word of mouth was so strong."

But it was a playground encounter that really brought the breadth of its popularity home to Bonneville.

"A kid in my son's school came up to me," he recalls. "He was about nine and said, 'I don't like what that Thomas [Downton's conniving footman] is doing'. I'd never, in a million years, thought it would appeal to those that young, but kids and grandparents are watching it together."

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The actor has been working for more than 20 years now, popping up as lovable buffoons in the romcom Notting Hill and period adaptation Mansfield Park, but it's Downton that's really made him a household name.

"I feel like a very lucky actor to be in a show that people love and continues to grow around the world," he says. "It's something I love doing, and the people involved are a fantastic team. I said to our producer not so long ago, that it's extraordinary to think that five years on, we're not punching each other!"

Series six has already been commissioned, but Bonneville is aware that it can't run indefinitely. "It's a unique show, in that it feels so big in scale, but it comes down to one person's imagination; [writer and producer] Julian Fellowes.

"It's really him, and he'll know when it's time to tie-up all the loose ends and put it to bed," adds Bonneville, who says he will be sunning himself "on a beach a long way away" this Christmas.

"Shows like Downton are once in a career, and I know it will come to an end eventually - and I can just retire and do some knitting."

One person who isn't so fussed about the series, however, is Bonneville's 12-year-old son, Felix.

"I've been filming Downton for six months of the year for five years now - that's half of my son's conscious life," says the small screen star.

"He comes to set and we stay over in a pub down the road and have a boys' night out. But I have to say, he's pretty bored of it now. But anything I do is the annoying thing that takes dad away from playing football."

And it has been a busy year for Bonneville. As well as reprising his role as the bumbling Ian Fletcher in W1A, Hollywood called with a role alongside George Clooney in war movie The Monuments Men - not that he's complaining about the shortage of time off ("I love the fact there's that variety").

And of course, he also filmed Paddington, which was released in November.

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"It was the script, as always," he says when asked what attracted him to the role of Mr Brown, the father of the family who takes in the young Peruvian bear with a passion for all things British.

"Paddington's such a vivid character for so many people of a certain generation, and within a page, I was laughing out loud and reminded of the charming innocence and warmth [author] Michael Bond put on the page. It was a no-brainer to be part of the film."

He's also reuniting with his Paddington co-star Sally Hawkins to film The Hollow Crown, a TV mini-series of adaptations of Shakespeare's history plays.

"We play the Duke and Duchess of Gloucester in Henry IV Part One. We're currently filming all over the country. Any medieval castle that looks vaguely appropriate - we're filming at it," he says.

And it won't be long before shooting begins on the sixth series of Downton. Interestingly, it's not the show people reference when the meet him, though. "More people come and talk to me about W1A than Downton," Bonneville reveals.

"Maybe they don't want to talk to the earl, but they want to talk to Ian Fletcher."

Downton Abbey will air on ITV on Christmas Day