When David Greig turned up at Sydney Opera House in 2012 as part of a tour of Midsummer, the ‘play with songs’ he wrote with Gordon McIntyre, frontman of Edinburgh indie-pop band Ballboy, he wasn’t sure what to expect. Here, after all, was a play with the lowest of lo-fi aesthetics and an Edinburgh-set rom-com with no budget that had been a huge hit on home soil four years earlier. The kooky charm of the show’s rollercoaster relationship between Bob and Helena was infectious enough for it to travel the world, and to travel light.

With only two actors and a couple of guitars onstage, despite the specifics of its Edinburgh setting, the universal appeal of Midsummer was plain. Here were two people old enough to know better, but who once they collide into each other over one mega-lost weekend can’t stop themselves.

Despite its success, Greig and McIntyre’s original production held on to its DIY aesthetic. Playing venues on the scale of Sydney Opera House, then with internationally renowned actress Cate Blanchett as co-artistic director of resident company, Sydney Theatre Company, was something of a surprise.

“We thought it would be in the studio,” Greig remembers. “Myself and the stage manager Sarah Scarlett turned up with a couple of poly bags, a ukulele and a plastic lobster, and I think Sarah had an acoustic guitar on her back. I remember us walking up to the actual Sydney Opera House, and we went to the stage door, and we were surrounded by all these huge trucks, and there we were with this little show, which ended up in a 500-seat theatre, and I remember looking out and thinking, wow. That’s one of the moments of my career I remember most.”

The National Theatre of Scotland’s new production of Midsummer that opens at Edinburgh International Festival may still be retaining its small-is-beautiful approach, but director Kate Hewitt’s reimagining of the play has expanded its horizons even more. Rather than having just two actors narrating Bob and Helena’s wild romance, here there will be four, with Eileen Nicholas and Benny Young playing sixty-something versions of the couple, whose younger selves will be embodied by Sarah Higgins and Henty Pettigrew.

As well as this, with McIntyre as musical director, a three-piece onstage band will feature doyens of Edinburgh’s fertile music scene, double bassist Clarissa Cheong of the band Eagleowl, Pete Harvey, who plays cello and piano with the likes of The Leg and Modern Studies, and actor/musician Reuben Joseph.

“I think we were all clear that we didn’t just want the show to be the same but with new people,” says McIntyre. “This time out it is going to be significantly different. Kate’s brought a lot of her ideas in, and we’ve spent a lot of time looking at how we can expand things without losing the tone of it. We’re all clear to it’s not a west end type musical.”

This has been the case from the day the first seed of Midsummer was planted when Greig was sent a mixtape by his friend, writer and academic Dan Rebellato. The collection included a track by Ballboy called Let’s Fall in Love and Run Away From Here. In an accompanying note, Rebellato described McIntyre’s melancholy lyric about a Russian lap-dancer and a fantasist customer as being akin to one of Greig’s plays.

“I listened to it,” says Greig, “and I thought, yeah. Then I bought everything I could by Ballboy, and became besotted with Gordon’s lyrics. When I realised he lived in Edinburgh, I thought, how can I engineer it that I meet this guy so I could talk to him about doing a show.”

When the pair met, the plan was simple.

“Two people, two guitars,” says Greig. “We decided we’d do everything you’d do in a big musical, but do it indie. We had this gang of people, and we had this one rule that if it isn’t fun and we don’t like it, then it goes. There was no financial risk, because we had no budget, and we both had plenty of other things going on, so we didn’t have to do it for anything other than our own pleasure. We weren’t trying to be successful, and it began with joy.”

Midsummer initially opened in the Traverse Theatre’s smaller space for ten nights as part of the short-loved budget-friendly Traverse Too strand. What was in part a love letter to Edinburgh was an instant hit, and crossed over to the theatre’s main space for an Edinburgh Festival Fringe run before touring the world in a way that led to Sydney Opera House.

“It’s ironic, isn’t it?” says Greig. “Your most successful shows always come from the idea that you’re not trying to be successful, but Midsummer was one of those rare and special things.”

When Midsummer first appeared, the idea of doing a lo-fi musical was unheard of. In the decade since, an entire generation of DIY auteurs weaned on spoken-word and music scenes as much as theatre has given rise to a mini genre of what is now known as gig theatre.

“Lo-fi was barely a thing then,” says Greig, “but these days you can barely move in a theatre without someone pulling out a ukulele.

As Midsummer comes home to roost in Hewitt’s reimagining of the show, it’s clear there have been changes in both Greig and McIntyre’s creative lives over the last decade. Greig is now going into his third season as artistic director of the Royal Lyceum Theatre in Edinburgh. His stagings of Touching the Void and Local Hero look set to be centre-pieces of the season. While McIntyre has continued making music as Ballboy, for the last five years he has also been head teacher of an Edinburgh primary school.

Theatrically speaking, the punky ethos behind the creation of Midsummer is a lesson that a new generation of theatre-makers are currently relishing.

“I think we hit it, not because it was calculated,” says Greig, “but because we decided to do something that we’d like to see, so it was a very happy accident. They don’t come along too often, and you’re never sure they’ll happen again, so when they do, you’ve got to look after them.”

Midsummer, The Hub, August 2-26, 8pm, except August 7, 14, 16, 21 and 23. August 16 and 23, 6.30pm and 10pm. August 11,18 and 25, 3pm.

www.eif.co.uk