Arriving onstage 25 minutes late, the trio take it in turns to offer an apology. Maybe they sense that the applause greeting the conclusion of each song is of inverse value to the huge crowd that has appeared to meet them. In all honesty though, this sedate response was more likely due to an awestruck mass unable to lift their hands to clap or raise their throat to roar.

The reason for this is no real mystery. Simply put; Hannah Reid’s pipes are an astonishing thing to witness in the flesh. The range is one thing, but the timbre is another. Delicate and sensitive, if not ever particularly fragile, she can elevate the wannest of melodies and arrangements into something of worth. It’s London Grammar’s ace up their sleeve. This is a superstar’s voice encased within an unassuming, polite and gentle young woman’s body. Slightly scruffy, she throws scraps of truth at the old aphorism, ‘never judge a book by its cover’.

Ably assisted by her cohorts, Dominic ‘Dot’ Major flits from keyboard to drums and never throws anything less than sympathetic colour to the group’s palette. Similarly, the simple figures cut out by guitarist Dan Rothman are enriched by a heavy reverb that drenches each note and chord. It is a mellifluous tone that creates the illusion of depth and belies the sparse, core setup of a three-piece. Occasionally accompanied by a string section sitting atop a giant LED block, the group douse the massive audience with songs of heartbreak and yearning.

Early on, Rothman informs that this is a festival that holds a special place for them. He claims that this where it all really started, and that it is from here that he can trace their trajectory, having played the smaller tent during the day previously. He implores that it is an honour that they now return to headline.

This tacit admission begs the question; one album in, is this a premature headline slot? The festival is young and so is the band. Can one keep the other afloat? Can the steep and rapid ascent of this group from Nottingham justify their status as a headliner? Such a thing occurred with the Strokes at Reading Festival in 2002 and the fullness of time has shown that they never quite scaled the dizzying heights of their debut again (at least, not to date). In London Grammar’s case, the evidence is at once compelling for their position and also slightly tenuous.

The main criticism against the group is the lack of diversity. They furrow a similar musical path as the XX; and it is a space that they seem to not only occupy, but dominate. Their forebears have been eclipsed. London Grammar fit the here and the now. They have enough to sustain a 50 minute set. For a more complete and nourishing experience though, they will need to branch out and explore different soundscapes, lest they reap inevitable diminishing returns come future releases. But, that said, this does not negate the powerful punch that their slim catalogue affords.

As an evocative ‘Hey Now’, ‘Flickers’and their cover of ‘Nightcall’ ripple pleasingly out across the night air, the penultimate song ‘Strong’ is introduced as their big breathrough crossover.

Unfortunately, Reid’s extended microphone, thrust out for the attendees to bellow lyrics back, doesn’t quite engender the communal eruption that she was hoping for, so, embarrassed and attempting to save face, she jokingly gestures the microphone towards Rothman to do the honours. He laughs and scoffs, which is a similar reaction as to that of the crowd. Those present didn’t stay silent because they wanted to; they stayed silent because they were daunted. Of that there is little doubt.

They conclude their main set with the dynamic crescendo of ‘Metals & Dust’. There is no encore. The old showbusiness adage is that you always leave people wanting more. And thus it was so here.

Their talent and execution is of the highest order, but their material is not quite on the same par. Time will tell if they can take the necessary leap to entrench their promising foundations, but withGrammar this good, they have the potential to surpass all of their peers. The fervent observers at Festival No 6 will gladly anticipate stellar graduation.

For now, London Grammar look like promising pups with bags of potential and plenty of star power in their locker.