Ah, Downton Abbey is back to reign over Sunday nights and – save your cynicism – add joy to our lives.

As ITV brings out its big guns for a fifth series, tuning in with a drink in your hand again feels like slipping on a perfect pair of PJs.

But how will Julian Fellowes’ costume soap (oh yes it is – don’t kid yourself) keep the adrenalin pumped after the last batch, which served up a rape, an unsolved murder and an illegitimate baby? With a winning formula of posh boffing (or the suggestion thereof), cutting put-downs, lots of hat-based dialogue segments and sinister facial expressions accompanied by swirly music, obvs. Mock it all you like (I saw someone on Twitter compare it to a bad dream about a Victoria Wood sketch – The Telegraph has a better class of troll) but that 90 minutes I smiled more than I had all week.

As the mistily familiar scenes of Downton slide back into view, we are constantly – and amusingly – reminded social revolution is round the corner. It’s 1924 and there’s a Labour Prime Minister (gasp), which causes our fave arch-Tory butler, Carson to fret about all he holds dear (serving tea to the aristocracy) crumbling away. “The nature of life is flux,” he tells Mrs Hughes, but she finds this “disgusting”. (Is that because she fancies him, fans wonder? They DID indulge in that shockingly saucy moment of hand-holding in the Christmas special...)

Lord Grantham (Hugh Bonneville) is back harrumphing about the house like a camp David Cameron, muttering how the grandchildren call him Donk (short for Donkey – there are less flattering nicknames, grandad) or how the sands of class and decency are shifting (casting quizzical eyebrows over shockingly opinionated women such as Tom’s new squeeze Miss Bunting). Luckily, Dowager Countess of Grantham is on legendary form as ever – whether she’s admonishing her snobby butler for pointedly withholding tea from Dr Clarkson (gasp) or delivering her now standard witty put-downs (“they’ve cast the net wide tonight”, she offers as an aside at seeing a teacher at the dinner table). She’s given something to do as well, which is refreshing. Her batty mate (Cousin Isobel/Penelope Wilton) is being wooed by a top toff and, just to make sure we know Maggie can’t stomach the notion of Cousin Izz becoming lady of all she surveys, the handy plot device of extra-dramatic swirly music is plonked on top. Granny gets busy sticking her oar in to stop this romance while reminding the viewer that she knows this is all a bit Mrs Bennet.

The joy of Downton is it is funny – sometimes intentionally, sometimes not. It keeps you guessing like that – but then is so well done that it tugs at your heartstrings. Lady Edith – a fairly frosty nob at the start – is likeable as a doting mum cuddling her daughter, who has been palmed off to the local farmer and his wife. After ITV’s Bafta success with Davina McCall and Long Lost Family, these scenes warm the cockles, especially when farmer Tom (in unintentionally hilarious fireman’s helmet) is reassuring her “we need a way for you to live the truth without telling the truth”.

As for sex, Downton is there to remind us how miserable life must have been if you had to do all your flirting in public, whether offering witty come-ons to suitors in front of your dad or while hunting grouse (Lady Mary) or hunting your prettyboy footman prey in cougarish style (Lady Antstruther/Anna Chancellor). What next: Carson and the Dowager indulging in some below-stairs heavy petting? This being silly, bonkers, delightful Downton, you never know.

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