How do you fill a vast arena with your music if it’s delicate, pared-down and gloomy?

The xx’s bass player and vocalist Oliver Sim says his three-piece have harnessed the talents of super-producer and remix king Jamie Smith.

Smith, of course, is better known as Jamie xx. He sat behind a laptop with Alicia Keys working on her recent single When It’s All Over, re-imagined tracks by Adele and made a steaming hot album of remixes with the late Gil Scott-Heron.

Sim says the band’s programmer and beat machine has upped his contributions to the shows after some of the band’s biggest solo dates – in Utrecht and in Antwerp – and before next week’s show at the Brighton Centre.

“He’s got a little children’s steel pan that we tour with,” reveals Sim. “He is pretty amazing on it. It’s the same one he recorded his Gil record with and his Adele remix with.”

Listen to the group’s second record, Co-exist, and you’ll hear the tropical sound open up Reunion.

“Jamie is one of those annoying people who can pick up any instrument and just start playing it first time trying,” adds Sim, who is backstage in Cologne writing new material when we talk.

Dubby house beats have been worked into Reunion and there are minimal club sounds behind much of Co-exist. It’s another way Jamie xx, who has become the name to book for self-respecting club promoters the world over, is pulling his weight.

“I was a fully-fledged groupie last year,” whispers Sim, as we pick up on Jamie’s DJ career.

“I was following him around to a lot of festivals, so was Romy [Madley Croft, guitar and vocals]. It was the three of us going to a lot of these DJ sets, in house nights and clubs, where Romy was DJing as well, and I can see how Jamie has changed the way he thinks.

“It’s something we are trying to incorporate into our music and Jamie has this mini village of equipment at the back of the stage he brings in, which is our moment to have fun in the set.”

The childhood friends, all now 23, are from Putney, south London. They met at the Elliott School, whose other alumni include Hot Chip, Burial and Four Tet (the successes are coincidence rather than connected). The eponymous debut album The xx, a sparse, melancholic record with an intent which defied its musicianship, won the 2010 Mercury Music Prize.

Not long after its release, Sim and Romy kicked out second guitarist Baria Qureshi from the band they formed in 2005.

Smith joined in 2006, but much of that first record was written by Sim and Croft.

“It’s one reason the new material is more honest,” explains Sim, as thoughtful and measured as his musical exploits.

“Writing love songs when you are 15/16 years old is a pretty funny thing because I wasn’t working off experience. It was a lot of peering in to other peoples’ relationships around me, and building on top of that my expectations and predictions.

“Now I’m 23 and my time has come and gone, it has been real life this time. It’s been a pretty cathartic experience putting pen to paper. Definitely I did some venting in songs. The first album was still a release, it was still my feelings, but it might not have been relationships I have been in.”

He will divulge no more. The band has been notoriously closed to the media since formation and the release of the first record. Some called it a crafty trick to magic some mystery. But perhaps it was youth or a lack of confidence in what they were doing – remember they won the Mercury Music Prize playing only what they were able to play.

“We are still the same band. I can hear that we still sound like us but our sound came about so naturally – a lot of the things people complimented us on we almost don’t want to take credit for because they came about like a happy accident.”

Part of the reason is he expected no one to hear the early songs. As such, Co-exist was a different beast to approach.

“A lot of the simplicity in the early songs came about because me and Romy were just learning to play instruments. We were 15 when we wrote VCR. It’s that simple because we weren’t capable to do anything more complex.

“And we don’t have the loudest voices so it wouldn’t make sense to make a huge sound we couldn’t contend with vocally.”

Thus the process is evolving. Already with an eye on the third record, Sim says at every venue they perform they set up a practice space backstage where they can write – either together or alone.

“For the first time ever we have been able to be creative on tour and we have started writing. That was my biggest problem touring the first record – I didn’t work at all. Now we are setting up a small studio space – guitars, keyboards, equipment to record – in each venue we go to.”

Yet the lyrics from the first record often refer to being unable to work or enjoy touring because his mind was on someone else, somewhere else in the world.

“I think it’s a combination of the two.”

So how did he feel opening his heart up on this record?

“Now I’m aware there is an audience I was pretty nervous before I started writing. We wrote most of the first album thinking no one outside the band would hear it.”

So they forgot about the world. They refused to share the music with their label and management. They took stock when the lyrics were being printed on the sleeves.

“That’s when I started to think about it. I didn’t freak out too much. I read interviews with people like Adele and I see how open she is about her songs and that she doesn’t mind sharing.

“But we don’t want to share everything and if I am thinking of people I like, I don’t want to know everything. I find it more interesting to make your own ideas. Right now if I wanted to find out Katy Perry’s height and favourite colour and the name of her dog, I could find it in ten seconds but that is not appealing.”

His advice for Alt-J, who will soon face the same daunting task of following up a Mercury Music Prize-winning debut, is to relax and enjoy the process.

“I remember being sat down by a journalist who told me that making the second record would be the worst experience of my life because the pressure was gonna be huge. He said we’d be constantly doubting ourselves, whether we should actively do something very different to the first record or stay true to our sound, and there would be pressure from the label and management. But I didn’t feel that at all.”

The decision they made was to go for continuity. Co-exist is subtle, reluctant, edited and crafted.

“I don’t think we have ever listened to a song and thought what could we add. It’s just a case of stripping away anything that doesn’t need to be in there.”

There is a sense with Co-exist that the band have become closer. Without all three members, The xx would not exist; it would be a different band.

“It wouldn’t sound like us. We only exist when the three of us come together.”

On the sleeve is another single letter X – this time filled with oil and water. And the 11 tracks develop thematically the idea that love, co-existence, is elusive.

“We want continuity in our work – we want a line in everything we do,” continues Sim, no doubt dressed all in black.

“The X gave us the freedom to do that and Romy came up with the whole concept of oil and water, which is how she came across the word Co-exist.

“I remember walking across the street once and there was some oil and water making rainbows on the ground. She did some research and found a line: ‘Oil and water don’t mix, they co-exist’.”

Such bittersweet sentiments are all- encompassing.

David Cameron and Samantha Cameron revealed they liked to cuddle to The xx’s music.

“I did hear about that. That is something I would really rather not think about.”

But The xx really is music for lovers, is it not?

“I’m happy people are getting down to it. But yeah, that’s a difficult one to think about.”

  • The xx play the Brighton Centre, King's Road, on Thursday, December 20. Doors 6.30pm, tickets £21.50. Call 0844 8471515