Marcus Mumford, lead singer/songwriter with rollicking folk-rock troubadours Mumford & Sons, is mindful not only of his own privacy but his bandmate Ted Dwane’s. “Can I tell the doctor story?” he wonders, addressing the chum perched next to him in a London hotel suite. “Go on, I’m not shy,” nods double bass player Dwane, happily.

“We were on an aeroplane back from a really long tour and I asked Ted, ‘Are you all right?’” remembers Mumford. “We’d all been drinking quite a lot, we were knackered, and he goes, ‘I just wanna go to the doctor, cos I want someone to hold my balls and tell me I’m all right’ [explodes with laughter]. My favourite story! I wanna tattoo of that.”

“It’s the most base level of a man’s needs,” smiles Dwane. “I’ve got a girlfriend now though…”

Since their 2009 debut album Sigh No More, the permanently on-the-road Mumford & Sons have achieved a remarkable feat: turned a group of whiskery, mild-mannered, waistcoat-wearing banjo dudes into one of the biggest bands on the planet – Brit Award-winning, Grammy-nominated sellers of five million albums worldwide.

Formed in London in 2007 through the “west London folk scene”, (including Laura Marling, who Mumford drummed for and dated), theirs is a raucous, rustic, hoe-down holler of often ecstatic bedlam.

Sometimes the best way to go about exploring a question is by writing a song

This year, in reaction to the planet’s now-humungous, homogenous festivals, they’ve curated their own one-day mini-festivals across Europe and America, and will soon do so in Australia, enlisting local bands, arts, businesses and food outlets – a downsized concept Mumford likens to “a Victorian travelling circus”.

Creative traditionalists to their vintage-wearing core, they’ve been spectacularly embraced not only by the global public but the global cultural establishment, having met (and played music with, or for) a preposterous roster of musical heroes and actual world leaders: they were Bob Dylan’s backing band at the Grammys 2011, recorded with Ray Davies in 2010 and played at the White House earlier this year at David Cameron’s personal request (for the Obamas’ PM-welcoming gala dinner) where they discussed Swedish folk duo First Aid Kit with Samantha Cameron.

“They’re definitely listening to BBC 6 in No 10,” cackles Mumford, unleashing his favourite word: “Awesome!” In November last year they played Sir Alex Ferguson’s 25th anniversary dinner, also at his personal request, and were waved on-stage from the wings by latest new fan Bruce Springsteen at Belgium’s Pinkpop Festival in May. The Mumfords, maracas in hands, sang Hungry Heart and went, frankly, mental.

“I’d never really danced on stage before. I couldn’t stop myself,” beams Mumford. “What you gonna do? Try and look cool? Fuck it, it’s The Boss!”

Suddenly, he looks aggrieved, contemplates The Big Issue’s Big List of Famous People The Mumfords Have Now Met (including their new pal Jake Gyllenhaal and porn star Ron Jeremy).

“Those were very fleeting moments,” he laments. “Names are so easy to write about, but I wish our crew members’ names were on there. They’re the ones we spend time with and who influence our lives. It’s been bizarre…”

Marcus Mumford, 25, a naturally bold character and the band’s acknowledged leader, has an air around him today of both agreeable warmth and arm’s-length caution, a man who married Britain’s beloved Bafta-winning actress Carey Mulligan in April this year, among the hay bales on a Somerset farm, the ceremony conducted by his vicar father.

Childhood acquaintances who attended the same church, their romance began in early 2011 after a Mumfords show in Nashville – a relationship Mumford has long vowed to remain protectively silent over (a vow his PR reiterates today), refusing to discuss even its tabloid-bothering impact, because any such acknowledgement only “furthers the impact”.

Curiously, though, he’s also clearly uncomfortable with ruminations on his faith. His parents are Christian church leaders John and Eleanor Mumford, founders of the Vineyard Church in the UK and Ireland, a Christian organisation and registered charity (which organises food runs in poverty-blighted neighbourhoods), while the band have many Northern Irish friends in the London City Mission. “We’ve been along to their breakfasts and they’re awesome,” notes Mumford. “And they always beat us at chess.”

When Sigh No More was released, featuring songs called Awake My Soul and Roll Away Your Stone, and themes of grace and denial, the Mumfords were considered vaguely spiritually minded.

Their second album, meanwhile, the Biblically-titled Babel (a broodier, moodier affair), positively pulses with religious imagery: “a brush with the Devil”, “I set out to serve the Lord”, “I was told by Jesus all is well so all must be well”, tales of “temptation” and “fickle flesh”. It seems Babel might be the official statement of their Christian faith.

“No, it’s not a statement of faith,” clarifies Mumford. “We don’t feel evangelical about anything. Really. Other than music.” A strained conversation ensues, Mumford deciding the language used “is more social than religious, verging on philosophical”. Suddenly, for the first time in your correspondent’s entire ‘showbiz’ interviewing lifetime, the word pluperfect comes up.

We don’t feel evangelical about anything. Really. Other than music

“The lyric you said, ‘I set out to serve the Lord’,” he notes (from the song Whispers in the Dark), “no one realises it’s pluperfect tense. The lyric is ‘I had set out to serve the Lord’. It’s looking back at a time when that had happened.”

So there’s themes, loosely, of your struggles with a Christian upbringing? A young Christian man who sold his soul to the rock‘n’roll Devil and perhaps has now come back to his faith?

“It’s not about that at all, sorry!” he hoots. “I don’t even call myself a Christian. Spirituality is the word we engage with more. We’re fans of faith, not religion.

“We’re just writing songs that ask questions. Sometimes the best way to go about exploring a question, things we wouldn’t necessarily talk about in conversation, is by writing a song. That’s why it’s quite hard… unpacking your songs. You write them in moments of privacy and… inadequacy. In articulation. When you can’t really express how you’re feeling, so you write it down with poetic licence and vent as much as you want.”

One song, Broken Crown, is astounding, a furious howl of indignation, Mumford hollering, “I’ll never be your chosen one… the pull on my flesh was just too strong… I hit the road and I fucked it all away” .

“It’s an angry one, that one,” he nods, unleashing his colossal, booming guffaw. “I’m never gonna tell you who or what it’s about though. Heheheh!”

“How did you get on with The Boring Two?” So cackles Winston Marshall (banjo) sporting an enormous beard (“It’s really pubey, I’m sorry”) and explosively fulsome hairdo, a look his chum Ben Lovett (keyboards/synths) describes as “a bouffant and a beard-ffant”. They’re strewn between cushions in the suite next door where they’ve just been discussing, “something very
inappropriate”, gay social connections app Grindr.

“So we’re A for anal and they’re B for boring,” chirps Marshall, jubilantly. Compared with The Boring Two, they are The Comedy Two, both preposterously handsome, charismatic young coves in vests and woollen togs who take little seriously. Lovett, he announces, has “brain damage, from the daily drinking” and is soon moving to New York after seven years of London-based musical life (for ever on tour, their base city makes no difference).

“It’s time to walk some different streets,” muses Lovett. “Meet some new people.”

Marshall may join him. “I just wanna be near him,” he swoons at his pal. “The Anal Team, together for life.”

Marshall, it turns out, isn’t remotely interested in faith (“I don’t really feel this stuff”) while Lovett offers an explanation for Mumford’s seeming vagueness. “There’s a reason why people write songs sometimes, because they can’t talk about something,” he decides. “So talking about the song you wrote, it fucks the point of the song that was written to avoid talking about it.”

Your correspondent has a general cultural theory: that this generation of 20-somethings, in an era increasingly dominated by vintage, traditional music, have vintage, traditional values. Two years ago, Adele noted her life’s greatest purpose wasn’t to sing but “to be a mum”, and yearned for the security of marriage. All her friends, she added, were the same, deciding, “I think we wanna be like our nans”. Mumford, certainly, is a family man (as is Mulligan, 27, who’s always talked of her love for kids).

“I think there’s truth in that,” nods Mumford, who hopes to tour, one day, with all their wives and children. “I hope so, that’s the dream. Like Blink 182, they tour with their kids. The road can be a destructive place. But if you’re careful it can be a wonderful place, a nurturing place. People are harking back.

“I guess vintage is cool. People are excited about things becoming smaller, buying their food locally, a shrinking in response to the ’90s probably. Where everything exploded. I think everyone wants something more comprehensible, in every aspect.”

The Comedy Two, meanwhile, blab about another of Mumford’s plans: to study agriculture at Open University.

“Marcus wants to be a farmer,” declares Marshall. “He’s always down on the farm, isn’t he? With his biblical farmer bullshit. I’m not interested! That’ll be the next album [claps hands, over-enthusiastic booming]. ‘So guys, uuum, birruv an agricultural vibe to this record, am I right? Eh? Eh?’ Bible in his left hand…”

Lovett: “Lamb under the other.”

Enjoy these glory days, boys. The band will split through agricultural differences soon enough.

Marshall: “You’ve sussed us out! I’m off with the rainbow flags on Grindr…”

Mumford & Sons

Mumford & Sons: “We’re fans of faith, not religion”