I SAY, is that a preposterously oversized stately home we see before us?

Is that a flapper dress? Is that Hugh Bonneville staring broodily into the middle distance? Oh joy, it must be ITV's Downton Abbey, back for another series, and we all know what that means. It means screeching poshies, surly domestics and impossible Americans, steam trains and black maria police cars, hats and fur stoles, hairdos more rigid than Lady Gaga's corsets and non-ironic use of the phrase "that little man in the village". For viewers, it means gleeful escapism and going around the next day trying to work the phrase "Oh, do be reasonable, mother!" in to ordinary conversation. If we're honest, it's this and its ilk - the period drama - that we pay our TV licence fee for.

We have had a lot of them recently, it's true. In fact, we've been up to our velvet pelmets in bewhiskered dukes and chambermaids with ideas above their station. In the last three years, more than two dozen costume dramas have screened, including Call The Midwife, Garrow's Law and The White Queen, and the wretched Village - and that's just on BBC One.

And there is more to come in the next few months: the final three Poirots, two more Marples and a return of Mr Darcy at Christmas in BBC One's PD James Pride and Prejudice sequel, Death Comes To Pemberley, starring Matthew Rhys (The Americans, Brothers And Sisters). And in 2014 we're promised Breathless, starring Jack Davenport as a dashing gynaecologist in the swinging 1960s; Our Zoo, about the creation of Chester Zoo; The Three Musketeers with Peter Capaldi; a Channel 4 follow-up to The Devil's Whore; a BBC adaptation of Daphne du Maurier's novel Jamaica Inn; and an ITV two-parter about Lord Lucan. A BBC version of War And Peace is planned for 2015.

There is also much excitement about a remake of romantic 18th-century melodrama Poldark (Richard Armitage is tipped to play the lead), as well as a shameless compendium of crowd-pleasing Victoriana in Dickensian, a remix of the 19th-century author's work in which his most famous characters from different novels cross paths. That's without even mentioning Julian Fellowes's The Gilded Age, an epic NBC network production about wealthy New York socialites set in 1880s Manhattan.

Some people moan about this -the actor David Morrissey for one, who reckons we should have fewer costume dramas on telly - but audiences clearly love spending Sunday evenings gatecrashing Henry VIII's weddings or chasing murderous villains through the streets of Whitechapel. The question is, why?

Who better to answer that question than Mary Gowlett, 66, a regular contributor to the fan website www.perioddramas.com, who is English-born but lives in the Netherlands. The former teacher believes viewers love these serials partly because they offer the opportunity to escape the stresses and strains of real, fast-paced modern life.

This could well explain the explosion of such dramas over the last few years, while the economy has flatlined and the future has seemed so uncertain. What could be better therapy than some immersion in the past? Yes, we see tragedy on Downton Abbey, including death on the Titanic and in the trenches, but none of that can hurt us now.

"Costume dramas make us put our own lives in perspective," says Gowlett. "We complain about the NHS, but we are so lucky we have medicine that can help us, and a justice system we can trust."

What starts as an escapist urge can quickly turn into fascination with the real history behind the fiction. In the absence of time machines, TV costume dramas are the closest we will ever get to living in the past (and accepting, of course, that Cesare Borgia, the amoral 15th century Cardinal of Valencia, probably didn't look like a teen idol).

This hunger to see the past up close is why the costumes and sets are so important. The epic shots of Highclere Castle in Downton Abbey are stately home porn; ITV's 1981 series, Brideshead Revisited (filmed at Castle Howard, in Yorkshire), had Americans practically fainting with desire.

It is probably no coincidence that Downton, Brideshead and Pride And Prejudice, the three international megastars of British TV costume drama, all featured imposing and dramatic stately homes. The success of Andrew Davies's 1995 version of Pride And Prejudice, for the BBC, probably owed as much to the location - Chatsworth House, in Derbyshire - as to actors Colin Firth (Mr Darcy) and the brilliant Jennifer Ehle (Elizabeth Bennet).

Audiences are captivated by period detail, particularly the clothes. Think of Lucy Honeychurch (Helena Bonham Carter) in the 1986 Merchant Ivory film, A Room With A View, with collars up to her chin, leather boots and long skirts flapping around her ankles even in 35C heat - no wonder she fainted in Fiesole.

Remember the scene in the garden where Lucy and her brother Freddy (Rupert Graves) are playing tennis in the hot sun, dressed in white, sleeves rolled up, while Cecil Vyse, Lucy's priggish fiancé (played by a very young Daniel Day Lewis) minces about in jacket and tie, reading through his wire-rimmed spectacles? Their clothing was a metaphor for the stifling social conventions that constrained them, discarded by Freddy and George when they go swimming, but clung to by Cecil.

And the actors. The joy of ITV's Poirot is that everyone is in it (after 24 years, there's barely a living thesp who hasn't got at least one episode on their CV) and it allows them to indulge in a bit of knowing self-parody. Tim Curry as a curmudgeonly archaeologist in Appointment With Death, Eileen Atkins as haughty Princess Dragomiroff in Murder On The Orient Express, Frances de la Tour as a writer of erotica in Death On The Nile: they're like a supremely talented am-dram group, hamming it up joyfully while the audience shares the joke.

It has to be said that there are some laughable anachronisms in costume drama, since there are certain cardinal rules of television that must not be broken. That's why you get Hollyoaks hair and perfect skin on every member of Henry VIII's court in BBC Two's The Tudors. That's why Anne of Cleves is played by the very pretty Joss Stone when Henry's fourth wife was reputedly (though probably unfairly) so challenged in the looks department she was referred to as The Flanders Mare.

We don't sweat that stuff, we costume drama addicts - at least, not within reason. It's just part of the trade-off. After all, a costume drama is entertainment first, history second, and handsome actors unravaged by the pox are what audiences expect.

Editors are interested in commissioning dramas that aren't too risky, and can produce a long-running brand, according to Glasgow University film and TV lecturer Ian Goode. Sumptuous looks help, as does pacey writing.

Downton succeeds, he suggests, partly because writer Julian Fellowes uses the classic literary serial tradition, but introduces popular soap opera techniques such as cliffhanger endings. Get it right and the rewards can be huge - Downton Abbey has been shown in more than 100 countries to date.

Goode speculates that period dramas also absorb us perhaps because they deal with issues rarely addressed by modern drama. Often the dramatic tensions are created by class friction, something that soaps used to address - remember James Willmott-Brown, the posh ex-army wine bar owner in EastEnders in the 1980s? - but rarely do today, even though class is still a live issue in Cameron's Britain.

Love across the class divide is a common theme. In Downton, the love affair between feisty Lady Sybil and Catholic Irish chauffeur Branson mesmerised viewers, and there is set to be more forbidden love in the new series.

Audiences expect romance and the more it challenges the mores of the time, the better, whether because of class, family feuding (Lorna Doone's romance with John Ridd), extramarital attachments (unhappy Irene's affair with architect Phillip Bosinney in The Forsyte Saga) or race (Small Island).

Isn't it time, though, for an epic Scottish costume drama?

"What has always puzzled me about drama production in Scotland is that producers stay away from Scottish literature like Sir Walter Scott," says Goode, adding that perhaps the success of Scott such as like Monarch Of The Glen, has tended to obscure the potential of the classics.

It is time that changed. Waverley, starring David Tennant and Karen Gillan: now that would be worth seeing.

Downton Abbey returns to STV tonight at 9pm