The first two episodes of Patrick Melrose (Sky Atlantic) have been dizzying high-wire feats – almost exhausting in their fabulousness. So it came as a surprise and a relief that part three of the Benedict Cumberbatch-starring adaptation of Edward St Aubyn’s quasi-memoirs played out as a more-or-less straightforward comedy of manners. Too much pirouetting and the blinding brilliance might start to feel like a stage trick.

Cumberbatch was barely in the second instalment, which focused on Melrose’s childhood in the South of France and the paedophile predations of his ghastly aristocrat father (Hugo Weaving, essentially playing Lord of the Rings' Elrond the Elf, if Elrond the Elf was on the sex offenders register).

As Cumberbatch returned to centre stage, the actor was more restrained – light years from the grim slapstick with which he had conjured portraying Melrose as a heroin addict in Eighties New York. It was 1990 and the traumatised toff was clinging to sobriety. Making tea in his grotty kitchen, he had to force himself not to chuck in half a bowl of sugar. Later, at a glittering shindig, he eyed the cocktails the way a great white eyes a shoal of mackerel.

He’d been dragged, kicking and squealing, to the birthday soirée of Sonny Gravesend (Tim McMullan) – one of the loathsome aristos writer David Nicholls and director Edward Berger have had such a jolly time skewering. “They’re the last Marxists,” whispered Patrick’s best pal Johnny Hall (Prasanna Puwanarajah) as they surveyed the titled twits. “The last people to believe class is a total explanation.”

Sonny was married to Bridget, the vapid hippy last seen taking pity on Patrick when he was a boy in France. Now, as played once again by an wonderfully jaundiced Holliday Grainger, she was a tragic social climber, initially oblivious to her husband’s philandering, and mortified by her middle-class mother (banished from the bash for fear of causing embarrassment).

Their guest of honour was Princess Margaret (Harriet Walter), a streak of bitterness and privilege who, as written by Mitchell, felt like a riposte to Netflix’s The Crown and its characterisation of the Queen’s younger sister as misunderstood party girl. Here she was a lemon juice in human form – caustic and snippy, with a face like a hatchet and a tongue like a rusted knife.

The instalment spun out as a game of existential Cluedo. Patrick was upstairs in the spare bedroom copping off with his old flame (married to a dolt in a kilt and also carrying on with Johnny). Bridget was upstairs too – about to walk out on Sonny over his just-rumbled affair with a glamorous American (pregnant with his desperately-sought male heir). Downstairs, Margaret wandered around being nasty – as did Melrose Senior’s old friend Nicholas Pratt (Pip Torrens, last seen playing Buckingham Palace’s Machiavellian fixer Tommy Lascelles in The Crown).

It was all terribly frothy – though the temperature plunged when Patrick confided in Johnny about his childhood abuse (the first person he’d ever told). Yet somehow the satire and the soul-searching were made to exist in perfect balance.

There was a broad hint, too, about where episode four is to lead, as Johnny told Patrick that what he needed was someone else to obsess over, rather than spend all day inside the four walls of his head. Funnily enough, Melrose had already felt a spark with Bridget’s cousin Mary (Anna Madeley) at dinner. The long arc of the series has been out of heroin hell and towards the light. What’s remarkable is that Nicholls and Berger have made the path to redemption as visceral and arresting as Melrose’s lost years with his demons.

Patrick Melrose is shown on Sky Atlantic, Sundays at 9pm. Or watch with a two week free trial of the NOW TV Entertainment Pass