There’s something in the air in London tonight: a sense of allure and anticipation as tangible as perfume and cigarette smoke in the night breeze, as sharp and explosive as a chemical reaction. On the South Bank, a hopeful queue for returned tickets stretches around the corner from the entrance to the BFI like a trailing scarf.

It’s cold and a bit damp, but the spirit of eager excitement is as indomitable as that of a crowd at the gates of a music festival waiting for their wristbands. Inside the venue, the atmosphere is even more charged. It’s as though everyone in the bar and the foyer, those with tickets and those with the lingering hope of picking up a spare at the box office, is on a promise; or at the very least, an extremely hot date.

In the auditorium itself, the identity of everyone’s preferred choice of scorching escort is clear. Flocks of Cumberbitches sit in patches of delight, flashbulbs exploding long before the star of the show emerges from the Green Room, whispers of ‘I actually saw him!’ and ‘He’s blonde at the moment!’ floating around the room like adulterous kisses.

Later, Caitin Moran, hosting the Q&A, has to conduct a synchronised squee to dissipate the sexual tension, which reaches feverish intensity when Benedict Cumberbatch finally takes his seat.

But of course, as for the Great Detective himself, the case is the most exciting thing of all. ‘Brainy is the new sexy,’ Irene Adler remarks – and A Scandal In Belgravia is brainy, sexy, heartrending, violent and funny; a treat for the head, heart, guts and groin all at once.

Of course, we can’t do too much kissing and telling. ‘Deny you were here tonight!’ co-creator Stephen Moffat warns afterwards, and to give away any of the most memorable moments tonight wouldn’t merely be a betrayal and a spoiler explosion – it would be a herculean task.

There are so many, from the very beginning to the final frames, that it’s like a rock band playing a set of new songs that sound like instant greatest hits: something brand new which feels intimately familiar, a fresh series of ideas and images that are immediately classic.

If the first series of Sherlock was The Smiths’ Hatful of Hollow, the second (judging by its opening instalment) is The Queen Is Dead, the knowledge of prior success making the production team even hungrier for more and creating a work of jaw-dropping magnificence.

What can we say without giving away too much, then?

Well, the reference to royalty isn’t accidental; the new case placed in front of Sherlock and John sees them taken to a certain palace, the good doctor helicoptered in from the countryside where he’s been doing the legwork and manning a webcam for his colleague, and Holmes himself dragged in from the comfort of Baker Street, wrapped in a sheet and nothing else.

The scene of the two of them, sat on a couch in a state room at the stronghold of sovereignty (‘Are you wearing pants?’ ‘No.’) is one of dozens of hilarious moments, with Cumberbatch and Martin Freeman laughing as much as you will. It’s like the line-up scene in The Usual Suspects, where the actors are corpsing as much as their characters.

Things get serious pretty quickly, of course, as the doughty duo head off in search of Irene Adler (Spooks star Lara Pulver) – a dominatrix known professionally as ‘The Woman’ – and the secrets she keeps locked inside her black and gold mobile phone.

The ensuing plot follows a typically Moffatish course through myriad twists and turns, up blind alleys, in and out of double- and triple-bluffs, with the real truth hidden, as ever, in the place you’ll never spot it: plain sight.

There’s global terrorism, street-level violence (everyone, even Mrs Hudson, gets caught up in the scrapings, smackings, nuttings, and throttlings) and the shadowy presence of that supremely evil hybrid of Raffles, Lex Luthor and Graham Norton, Jim Moriarty (Andrew Scott) hanging over the proceedings like the London fog which used to blot out so many of Holmes’ original adventures back in 1895.

There’s also a lot of nudity, which didn’t appear much in the nineteenth century, and a thread of something that is definitely romantic but not exactly a romance.

‘Something happens,’ Mark Gatiss says after the screening, ‘but it doesn’t have to be something as mundane as a love story. It’s more interesting than that.’

‘Our favourite film is The Private Life of Sherlock Holmes,’ Steven Moffat adds (and that’s not just because Gatiss’s inspiration for his performance as the detective’s brother comes from one of its stars: ‘I owe it all to Christopher Lee – he was the first thin Mycroft’), explaining Irene Adler’s spiky character comes from ‘my lifelong obsession with difficult women.’

‘She’s very flawed, damaged, fearful,’ Lara Pulver admits of her character. ‘She has a wonderful mask that’s solid and then she meets this man and … it’s like looking in a mirror.’

The panel also talk about the past and the future of Sherlock. ‘We thought it would be well-reviewed and prestigious,’ Moffat says of the first series, ‘but we didn’t know it would be so whoppingly huge.’

As for the second – which also features The Hounds of Baskerville (Moffat: ‘There’s got to be a dog … probably’) and The Reichenbach Fall (‘The big, huge battle between Sherlock and Jim Moriarty’) – was there any reason for choosing the biggest and most famous of Arthur Conan Doyle’s stories and shooting them all at once? ‘Why wait?’ Moffat shrugs. ‘To hell with deferred pleasure.’

Of course, the pleasure of seeing the new series of Sherlock must be deferred for a little while – A Scandal in Belgravia doesn’t air until New Year’s Day – and our lips are sealed as to any further details until then. But as all lovers are aware, anticipation and secrecy are part of the pleasure – the rest is just chemistry.

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