Last November, for the 25,000th time on the stage, the actor playing Sergeant Trotter in The Mousetrap stepped forward during the curtain call and asked members of the audience not to reveal the play's surprise ending to others. To do so would, by implication, spoil the whodunnit for future audiences. Over the years the odd clever-clogs stand-up has disobeyed the injunction. And whoever wrote the play’s Wikipedia entry also gives the game away.

I was going to make a strenuous effort not to give away the ending, but since it's all over the front pages of the newspapers there's not much point. This rambling Downton special spent two hours going nowhere in particular, albeit very charmingly, but Julian Fellowes had been keeping his knuckledusters hidden behind his back. In the closing few minutes, he gave us the new heir of Downton and got rid of the previous one, the much-loved Matthew Crawley.

Julian Fellowes has often seemed to treat Downton Abbey as a speed-writing contest, with momentous events and the tide of history whirling past like roof tiles in a typhoon. Happily, as series three has developed, the pace has evened out a bit, though Shirley MacLaine's Mrs Levinson barely lasted as long as the disfigured pretender to the Downton inheritance in series two, while the storyline in which Downton was financially ruined and then promptly saved by Matthew's convenient inheritance was straight from the Peter Pan book of screenwriting.

You suspected she was a goner the moment the doctors started to front up like King Kong and Godzilla. Having given birth to a girl, the rebellious bluestocking Lady Sybil got her marching orders last night on Downton Abbey and Jessica Brown Findlay’s husky larynx will be heard no more pouring oil on troubled waters. The rest of the cast can rely on a berth in Julian Fellowes’ gilded prison for all eternity. Ms Brown Findlay is available for work.

Mid-September: the nights are drawing in and, to quote that well-known costume dramatist John Milton, the period detail is as “thick as autumnal leaves that strew the brooks in Vallombrosa”. Downton – praise be! – is back. However, before the third series even gets into its penguin-suited stride it seems that paradise is about to be lost all over again. Lord Grantham has blown most of his wife’s fortune: the Canadian Trunk Line Company has hit the buffers and gone bust. 

Imagine my surprise when we weren't much more than halfway through this first episode, and the flipping thing hit the iceberg. But of course writer Julian Fellowes was way ahead of me, and his four-part series about RMS Unsinkable is evidently going to circle around the vessel's fate from various viewpoints in assorted time frames.

Though the wind had wailed mournfully through the plot-holes of the second series of Downton Abbey, writer Julian Fellowes was in his element for this two-hour Yuletide spectacular. With the characters in place and a cluster of storylines tantalisingly in play, it boiled down to a grand game of tying knots, building climaxes and sawing off the loose ends. Framed as a Christmas shooting party with a grand gathering of friends, relatives and prospective in-laws, it was the Gosford Park of the Downton canon.

ITV has been cunningly trailing its Christmas bumper edition of Downton Abbey, which will feature guest stars Nigel Havers and Samantha Bond and the spectacle of Mr Bates being dragged before the beak for murdering his first wife. Now that details of the Yuletide schedules have emerged, it's clear that Downton is the one to beat on Christmas Day.

And so the eventful second series surged to a close with a bumper 90-minute edition - or at least it was in a 90-minute slot, generously padded with the commercials battling to scramble aboard the great ship Downton - and we were still left dangling in Mary and Matthew's will-they-won't-they neverland. The show's resemblance to a gargantuan soap which has been telescoped into a handful of Greatest Hits episodes was never greater.

I suspect writer Julian Fellowes's guilty secret is that he has an attic stuffed with novels from Mills & Boon, such are the luridly romantic plotlines and cliché-flirting characters in Downton Abbey. If you think you can see it coming, then you probably can.