Downton Abbey, Series 5 Finale, ITV

On and on the stately galleon sails. The fifth wodge of Downton Abbey has been light on utter knuckle-gnawing preposterousness. Plots conjured up at random from thin air have been in slightly shorter supply than usual. The very worst you can say of it is that Lord Fellowes is no Agatha Christie. The poor old blighted Bateses have now been subject to a matching pair of cack-handed murder mysteries.

Last night brought further proof that the upper classes are just as capable of implausible bluntness at the dining table as that uppity pinko teacher whom Tom the former Fenian had his eye on. Lord Merton’s sons were a pair of frightful bounders to their prospective stepma Isobel (Penelope Wilton) last week. This week Rose’s comically ghastly mother Lady Flintshire (Phoebe Nicholls) turned feral attempting to thwart her daughter’s acquisition of some posh Jewish in-laws. “Do you have any English blood?” she drawled. This was Downton tackling that great contemporary shibboleth, multiculturalism, as only it can.

Spratt the Geordie butler who is the most crashing snob of them all should be given his own sitcom

And yet we are bound to Downton Abbey by hoops of steel. This is the greatest mystery of all, far more than the long-milked plot about who pushed Green the rapist under a Piccadilly omnibus. Downton is full of many elements that verge on incompetent. How can it be otherwise? There are too many balls to juggle in the hour-long slot, lengthened last night by half an hour. So many plotlines rely on one character accidentally overhearing another. Characters continue to act in motiveless malignity, while others are well-mannered to the point of vapidity. This series Lady Mary (Michelle Dockery) has turned inexplicably back into a bitch. To cap it all, last night the Dowager Countess (Maggie Smith) was being invited to roll in the hay with her Russian admirer.

And offstage, somewhere in the wings, history continues to be beastly. Lady Edith – is she a magazine editor, or isn’t she? – lost her babyfather to those nasty chaps in brown shirts in perhaps the naffest storyline of all. Last night it was the turn of India to supply the noises off. “We heard about that terrible Amritsar business,” said one lord to another lord with a third lord listening in.

Along the way, though, there is plenty of perfectly good pathos – Mrs Patmore’s moment in the spotlight at the new war memorial was nicely done, and it neatly explained why Isis the dog needed to be killed off (Lesley Nicol pictured). And there is always good serviceable comedy to enjoy. Hugh Bonneville lamping an overtanned Richard E Grant was a laugh, and Spratt the Geordie butler who is the most crashing snob of them all should be given his own sitcom.

Next stop Christmas. Treats to look forward to include Tom shoving off to Boston, Lady Edith’s sprog being exposed and how about Molesley (the excellent Kevin Doyle) going down on bended knee? He’s been a treat this series. And Thomas (Rob James-Collier) can’t play good footman for very much longer. Only the autodidactic Daisy (Sophie McSherra) will stay exactly where she is, because that’s what she does.

Not long to wait. Last night’s commercial breaks were full of seasonal retail opportunities. There was even an ad for the Downton Christmas LP with turns from the stars. If you buy that, you are either suffering from toxic levels of irony or you are even more enslaved to Lord Fellowes than the rest of us.