"As a father of twins and a diplomat, I usually avoid the word favourite," says Matthew Barzun. "But you're about to hear my favourite band." The 43-year-old United States Ambassador to the United Kingdom is standing on a makeshift stage in the entrance hall of Winfield House, the neo-Georgian mansion set in 12.5 acres in London's Regent's Park, which since 1955 has been the official residence of whoever holds that title.
Upstairs is the bedroom where President Barack Obama stayed during his last state visit to Britain in 2011; on Friday night its occupant was Matt Berninger, the lead singer of American band The National. "I expect there'll probably be a pack of cigarettes, some dirty magazines and the launch codes under the mattress," he says, before leading his band on stage for a nine-song set.
Welcome to the Winfield House Sessions: a series of intimate concerts featuring American and British acts that Mr Barzun has been organising since he became ambassador in August last year. The two previous gigs were by Damien Jurado and Ed Sheeran; clapping along to The National were actor Mark Ruffalo and artist Sir Antony Gormley.
On the surface, the evening appears to be the antithesis of the traditional black-tie reception: the dress code is jeans; buckets of beer stand on elegant antique tables and the ambient music is a thumping, nightclub beat. But every detail has been carefully thought out, from the canapés (fish'n'chips, corn dogs and beef tacos) to the cocktails (Old Fashioneds or gin and tonic).
It is a masterclass in what Mr Barzun calls "cultural diplomacy". "The contrast of the beautiful, formal setting with more informal music and wearing jeans is part of the appeal," he says before the concert, sitting in his office at the US Embassy on Grosvenor Square.
"That term 'cultural diplomacy' is a really powerful one, because there's not agreement on what it means. There's a group of people I'm sure who would be like: 'Oh come on, that's an oxymoron – real diplomacy happens behind closed doors, with government officials.' But another school of thought would be: 'Can you imagine being effective as a diplomat without culture as a part of it? That's how anything and everything happens'. "
Mr Barzun is not a career diplomat: he used his tech background from his time at CNET Networks to great advantage by organising small-scale, grassroots fundraisers with an online presence during Mr Obama's presidential campaign. He was rewarded with an ambassadorship to Sweden in 2009, his only previous posting.
There is a giant poster of Johnny Cash on his office wall, next to portraits of President Obama and Winston Churchill.
Mr Barzun says he loves Cash because his sound is "uniquely American", a fusion of rock'n'roll, blues and country. "I have him, and Winston Churchill and President Obama keeping an eye on me. That's a good line-up, a good set of moral compasses," he says.
To get things done, he "colour codes" his days, using stickers for certain subjects. Blue for "government-to-government stuff", green for trade, yellow for the media, grey for internal meetings and purple for "engaging in new ways", under which the Winfield House Sessions fall.
He says he was inspired to open up his official residence by the Obamas. "They continued the wonderful traditions of state dinners, that sort of thing that the White House has always done, and they introduced new ways of bringing people together, and opening up, and welcoming through music."
David Cameron and his wife Samantha were seen nodding along to Sheeran when he played Winfield House in April, but Mr Barzun reveals that he and the Prime Minister have only recently come to know the singer's music. "We had been together at Chequers playing rounders – badly on the Barzun side," he says. "And we were talking about music … we learned about Ed Sheeran through our children and now we've become big fans ourselves. It was nice for them, I hope, to have a night away and just enjoy great music."
Mr Barzun went to Glastonbury with his son, tweeting pictures of his muddy wellies. He says he "only recently" cut off his wristbands and that his son is still wearing his. "At some point my wife's just going to have to snip them off while he's sleeping," he jokes.
Some might say that he's on dangerous ground, given British politicians' troubled history with pop music. Gordon Brown's professed love of Arctic Monkeys was the subject of much ridicule, while Mr Cameron was famously "forbidden" from liking The Smiths by the band's guitarist, Johnny Marr. But the ambassador is hurt by the suggestion that bands wouldn't want to play at Winfield House; none of them are paid to perform.
"There's no arm twisting," he says. "I think if it were perceived to be something manipulative, people would start saying no."
Matt Berninger admits that the appropriation of music by politicians is "obviously something that happens", but adds: "That's not what's going on with Matthew. I think Matthew is kind of a big music nerd."
For the ambassador, compliments don't come much better than that.