Julian Fellowes’s series was destined to divide opinion from the first shot of its opening credits: Hugh Bonneville’s Earl of Grantham strolls towards his country pile, steadfast labrador Pharaoh (the unfortunately named Isis came later) trotting alongside while a string-drenched theme tune blarts away over the top. The name of that theme: Did I Make the Most of Loving You? I know, I know.

Yet there was more to Downton than met the monocle. Those who dismissed it at face value missed out on a first series groaning with mega-budget magnificence, carefully researched stories and sizzling dialogue, delivered by a fine cast of young bloods and old hands anchored by Maggie Smith and her imperious mic drops. It was sentimental, camp and daft, certainly (this was a drama soapy enough for a vindictive housemaid to slip a bar of the stuff under the foot of the lady of the house, causing her to fall and miscarry). But it was also touching, funny and involving: Gosford Park without the distraction of a whodunnit.

But a second season made me rethink an argument to which I had stuck as faithfully as Pharaoh to his master’s side. The problem was war, which, if nothing else, was good for exposing the perils of giving someone a few months to bash out the sequel to a first series they’d spent years planning.

Matthew Crawley – dashing, honourable heir to the Downton fortune, and played with admirable dignity by Dan Stevens – had taken some shrapnel at the Somme and returned home paralysed from the waist down. Having spent half the series understandably despondent, he was all set for a genuinely interesting and emotionally powerful exploration of adjusting to life as a paraplegic in the early 20th century.

But not for long. First, Matthew felt “a tingle” (whether it was in his legs or his love pump was left discreetly unclear) and then, appalled at the prospect of his fiancee dropping a tea tray, leapt to his feet to steady her. Muscular atrophy be damned! By the end of the series, he was walking; by the start of the next, he was bonking. The paralysis, it transpired, was in fact spinal bruising, medically possible – just – but narratively insulting. No one expected My Left Foot, but neither did they expect disability to be reduced to mere plot device. Downton had achieved the seemingly impossible: flunking the dramatic potential of the first world war. From this point on, Fellowes stopped trying to write credible drama and began to openly take the piss.

Before the series was out, we’d had a bandaged, amnesiac, Canadian casualty of war claiming to be the true heir of Downton; the dependable Earl of Grantham inexplicably snogging a maid; and the Irish Republican chauffeur eloping with comely Crawley daughter Lady Sybil.

After this, it became a chore: bogged down by motiveless behaviour, endless exposition, a badly mishandled rape and a cackhanded scene at the house-versus-village cricket match, during which the aforementioned chauffeur was finally welcomed into the family after taking a vital catch, filmed in slo-mo. It was enough for the millions of loyal viewers worldwide, but so much less than it could have been. Alas, Downton, I did not make the most of loving you.