Simon Kelner:The Abbey habit has been ruined by advertising

It was one of the most keenly anticipated television events of the year, watched by a massive audience who, it seems, just can't get enough of being reminded of timeless virtues and a rather twee view of the world. There were period dresses, perfect manners and an interesting juxtaposition of old and new. The acting was good, even if the storyline was a little predictable.

Whichever way you looked at it, this was a blockbuster, the latest in a long line of successes. Yes, the new John Lewis advert – which viewers of Downton Abbey will have seen twice on Sunday – was a characteristically sweeping piece of work. It was a split-screen romantic fantasy involving a girl from the 1920s and a modern-day boy. She wears Mary Janes, he wears trainers; she writes on vellum notepaper, he uses an iPad; she works in a telephone exchange and he looks as if he is in a design studio.

Anyway, you get the idea: it's an age-old boy-meets-girl story, imagined in a way to suggest that, however our lives have changed, John Lewis remains a constant, reassuring, presence. "What's important doesn't change," ran the self-regarding message at the end. Never knowingly understated, John Lewis advertising is perfectly suited for Downton Abbey, whose audience, presumably, had already bought into a theatrically old-fashioned view of the world.

There was so much advertising around the programme that it was sometimes hard to know where one started and the other finished. You couldn't let your attention wander for a second, otherwise you'd hear Lord Grantham learning that his family fortune had been lost, and that he might have to sell Downton, and the next minute you'd be hearing a voice saying that the Halifax Building Society could help you "find and buy your perfect house". The casual viewer could have found it all very confusing.

There were ads for home insurance, and I found myself thinking that if only Lord Grantham had called Direct Line, things may have been very different. The plot lines for the ads were generally less predictable and more thought-provoking than those of the programme itself. I was particularly taken with an advert for a dog food which offered your canine companion "lightly grilled chicken". Now, I don't know about your dog, but I've never heard mine complain about the precise way in which his supper has been cooked. "I'd like my food to be steamed from now on," he wouldn't say. "Much more healthy. And tasty, too!"

But what about Downton itself? It was as perfectly potted as a tin of Kennomeat, and contained all the ingredients for an autumn of Sunday night viewing, once I get over the fact that I still think of Hugh Bonneville not as the good Lord, but as Ian Fletcher, the head of Olympic deliverance from the brilliant show Twentytwelve. I half expected him to greet the news of his financial disaster with "So, that's all good then."

And I know it's on ITV, but it surely blurs the line between programming and advertising too much when the announcer says over the closing credits: "You see, it was great, wasn't it?" I think we'll be the judge of that, thank you very much.