Now a small Scottish town is gearing up to cash in on the Downton Effect, with an episode of the hit ITV costume drama filmed at Inveraray Castle expected to be one of the highlights of this year's festive viewing.
The cast and crew of Downton Abbey travelled to the royal burgh in Argyll and Bute earlier this year to film scenes which will air as part of a two-hour special on Christmas Day.
The show is mainly known for being filmed at Highclere Castle in Hampshire, the home of the Earl and Countess of Carnarvon, where British and American tourists have flocked since the series became a hit in 2010.
On Tuesday around ten million people are expected to tune in to watch the festive special, which will see Lord and Lady Grantham and their family heading for a summer break at fictional Duneagle Castle to visit cousins in the Highlands, while most of their servants remain at home.
In real life, Inveraray Castle is home to the Duke and Duchess of Argyll, who will be settling down to watch it on Christmas Day in expectation of the ancestral home having a starring role. Torquhil Campbell, the 13th Duke of Argyll and chief of the Clan Campbell, told the Sunday Herald he hoped it would boost the 75,000 visitors who currently come to the castle every year.
"That was the reason we did it, to promote the castle and bring in tourism," he said. "You have got to keep trying new ideas. Downton Abbey is not just watched by British people, it is incredibly popular in the States, so there is quite a wide audience to attract."
The filming took place while the Olympic Games were on in July and August – the only time enough bedrooms could be secured in the Inveraray area for the cast and crew.
Unfortunately, it also coincided with the time the Duke, 44, was on an annual break in Tiree with his wife Eleanor and their three children.
"We go there for the same two weeks every year and had invited lots of friends – there was just no way we could have changed it," he said. "It was devastating having put all the work into to it and to suddenly find out the only time they could do it was the time we weren't here."
The Duke and Duchess got daily updates on the filming, which involved 20 to 30 articulated trucks rolling into the estate and the film crew working to ensure any clues to the modern world – such as electricity sockets – were hidden.
Among the areas of the castle used in the filming were the armoury hall, which boasts an impressive display of muskets, axes and broadswords, and the elaborately decorated state dining room.
The couple have yet to see how much of their home will make it to the final cut, and have no idea of any of the details of the closely-guarded plot.
But the Duchess, 39, said she hoped they would be able to give any Downton enthusiasts a flavour of the filming when the castle reopens to visitors after winter from March 29 next year.
"We will talk to guides and if people want more of a Downton tour then we can arrange that," she said.
The first series was set in 1912. It has followed the fortunes of the aristocratic Crawley family through to the 1920s. In that decade, Inveraray Castle was inhabited by the 10th Duke of Argyll, Niall, who built the Bell Tower in the town to commemorate those who died in World War I and was a scholarly recluse later in life.
The Duchess said: "He was brilliantly clever, never got married and very, very eccentric. His mother slept under a fishing net and talked to the 'wee folk' and his sister, who also never got married, lived here [in the castle]. He used to have an organ on the balcony and used to sing for his guests."
While life at Downton depicts more than a dozen servants looking after the Crawley family's every whim, the life of the aristocrat in the 21st century is not quite the same.
As well as opening Inveraray Castle up to visitors for more than half the year, the 60,000 acre Argyll estate earns income from diverse activities such as forestry, agriculture, stalking and holiday cottages.
The Duke said: "At the end of the 60s there were around 30 staff living in the house. Today we have a caretaker and cleaner living in the castle – and us. Yesterday I went off to chop down our Christmas tree.
"It is very different and we are obviously much more hands on when it comes to the castle – my wife runs the shop and the tea-room."
More than 100 countries have bought the rights to broadcast Downton Abbey, which has won a clutch of awards and had three Golden Globe nominations earlier this month.
The popularity of the series is said to have boosted sales of large period properties and even sherry, which the cast often drink.
The show's makers are now reportedly looking into bringing out a range of branded products such as homeware and kitchen items.
Meanwhile a rise in the number of babies given names such as Violet, Isobel, Isabelle, Daisy, Matthew, Thomas and Tom in 2012 has been put down to the "Downton effect".
For some American fans, the only way to watch the programme is at a viewing party, with a proper cup of tea and a finger sandwich in hand.
In the UK, locations such as Highclere House and the village of Bampton, Oxfordshire – where exterior shots are filmed for the show – are now a huge draw for fans.
Lewis Swan, director at Brit Movie Tours, said the impact of the series had been huge.
"These places are now getting coachloads of people going along to them," he said.
"People love all the glamour and fantasy that Downton projects. We have got a lot of people coming both domestically and from overseas countries such as American and Australia."
Swan said there was "no doubt" that Inveraray Castle would receive a boost from being featured in the series. "It looks a spectacular location and it will be a nice inclusion to the world of Downton Abbey, having a Scottish location like that," he added.
PRODUCERS say the fictional Duneagle Castle is a grander step up from Downton Abbey and the family will be seen stalking deer and fishing in the surrounding countryside.
John Patrick of leisure firm Argyll Adventure was an extra in the production, along with some of his staff and his horse, Lewis.
He said: "We were originally asked to provide animals but at the same time they asked if we had a couple of chaps to help out as extras."
He added: "Everyone was talking about it and we'll all be glued to the telly on Christmas Day. Inveraray Castle is obviously our jewel in the crown up here."
There has already been a glimpse of equine star Lewis, selected for looking like a traditional servant's horse of the period, in the trailer for the Christmas Day episode.
Rachel Foster, 18, a ride leader at Argyll Adventures, said: "We weren't allowed to say at the time what it [the filming] was, but Lewis is one of our main horses, so everyone was excited when we told them.
"You can see his silhouette in the background for the trailer. I think it might help to bring tourists into the area."
In Inveraray, other locals hoped the drama would boost tourism, but pointed out it was already a popular spot for film crews and visitors.
Julie Mackenzie, receptionist at the The George Hotel, said projects filmed in the area ranged from the 1990 film Bullseye starring Roger Moore and Michael Caine, to the infamous scene in Eastenders where Janine Butcher murdered Barry Evans by pushing him off a cliff, shot at the Rest and Be Thankful.
She added that Maggie Smith, who plays Dowager Countess of Grantham, had stayed at the hotel during the filming.
Donna Heron, manager of Brambles restaurant, said the town was already an extremely popular destination in the summer.
She added: "Of course any extra visitors would be welcome."
Last year a survey by the motoring and leisure association CSMA found a rise the number of "TV tourists" who visit places associated with their favourite television shows.
As well as Highclere Castle – where visitors were predicted to rise 400% last year – other popular destinations include Barry Island in South Wales, the setting for Gavin And Stacey and Being Human.
Barbara Clark of VisitScotland urged local businesses to take advantage of the Downton phenomenon.
She said: "Downton Abbey is about the biggest thing on television at the moment.
"We would urge businesses in that area to get information out about it. It's a huge opportunity for them to promote the area."
Clark said overseas visitors were often inspired by their choice of viewing to visit parts of the UK.
"It's a phenomenon known as set jetting," she said. It's a growing trend of film and TV fans visiting locations of their favourite movies and TV shows.
"It's not just the blockbusters like Brave, it's also things such as Monarch Of The Glen, Downton Abbey and Hamish Macbeth.
"These sorts of shows absolutely couldn't be better adverts for Scotland in terms of scenery and the people, whether they're real or animation. They inspire people to come to Scotland."