Sarah Sands: 'Downton Abbey' is sloppy tosh. That's why we love it

What a terrible weekend for the BBC. Not only did strike action take from us the Today programme, Front Row and other middle-class pleasures, but it's been beaten in the field it used to call its own. The most perfect middlebrow drama this year concludes tonight – on ITV.

Downton Abbey is the answer to the eternal question of what makes us happy: a costume drama on a winter evening. We knew this from Cranford, but that was an unsurprising triumph, for we are primed to expect such things from the BBC. The success of Downton Abbey stole over us like winter mist. Even as astute a critic as Rachel Cooke wrote off the first episodes as creaky and preposterous. TV folk were busy fussing over Saturday night audiences. They could only envisage family showtime and tears.

What older audiences yearned for was stiff upper lip and a bit of action below stairs. There were other, unexpected pleasures, such as spotting mistakes. Detecting Edwardian TV aerials and double yellow lines became a national parlour game. Julian Fellowes, the writer of Downton Abbey, who has developed a Maggie Smith-style ability to take offence, attributes the carping over period detail to left-wing people "who are insecure socially". But he can't have noticed the hue and cry in the Telegraph over social solecisms. It is not peculiar to the left wing.

Charles Moore wrote in The Spectator that it was bracingly bold in these times to make the Earl a goodie and the gay footman a baddie. Richard Dennen, in the Evening Standard, mused that he too had enjoyed rogering a butler while staying at a relative's Scottish castle.

Meanwhile the "socially insecure" enjoy the servant plot lines. Not just because of Bates-lust. Housework is a lapsed art, which we regard with mystified fascination. When the National Trust advertised a housekeeping course, my career-minded friends squealed and fought to sign up to it. I was as intrigued by the servants' rooms as the wicked aristocrat in search of incriminating letters.

The National Trust has offended some purists by offering an "experience" of famous houses. Instead of filing past a rope, visitors can sit down and read books and imagine themselves into the life of the house. Downton Abbey is a television equivalent.

What of the plot, chunks of which have been allegedly lifted from Little Women and Mrs Miniver? Well, when Russell Davies drew on the best storylines in history for Dr Who, we applauded his ingenuity.

These are the safe ingredients for blockbuster television and, indeed, all drama; a house, a letter, a disaster, an unacknowledged love, a lost fortune, a stolen object and a person wrongly suspected, a hero/heroine feared dead but nursed back within one episode, a secret and war on the horizon. Fellowes has chucked all these in and the result is delicious.

There have been other theories on the particular alchemy of Downton Abbey. The Edwardian period is far enough away to look like history, but not so far that we don't want to live there. It is also bang on the school history curriculum. It has made or rescued the careers of many of its actors. It has raised a puzzled ITV into BBC heartland. It has changed the dynamic of our weekend. How shall we manage tomorrow?

Sarah Sands is deputy editor of the 'London Evening Standard'