The writing of Theatreland's oddest musical

How do you write a song about serial killings? Adam Cork tells Matt Trueman how London Road came about

New musical hunting is an alchemical sport. Producers come up with some unlikely combinations hoping to spark public interest. A Greek island romance with Abba songs? Jerry Springer: The Opera? Even so, when the National Theatre announced London Road – about the 2006 Ipswich serial killings – eyebrows were raised. On paper, it looked obscure, even downright wrongheaded. Yet, a year after its sell-out run in the Cottesloe, with a Critics' Circle award to its name, Alecky Blythe and Adam Cork's musical is being upgraded to the National's largest space, the 1,160-seat Olivier Theatre, during the dreaded Olympic period.

Not bad for a project that came about by fluke. In 2007, Cork was one of three composers on a National Theatre studio workshop week, and Blythe one of three writers. They paired up and, since neither had heard of the other, Blythe started by explaining her distinctive working method, which involves actors replicating the exact speech patterns of interviewees, down to the last "um" and "er".

"There was never a god-like figure who said, 'Aha, Adam Cork, Alecky Blythe, verbatim musical. That's what we'll do'," explains Cork in a quiet corner of the National's café. "We didn't know anything about each other … but the more I heard, the more I thought, 'Gosh, yeah, if one was absolutely fascistic about the documentary detail of the voice in a musical sense, there could be something really exciting here'."

The intellectual challenge appealed. Cork read music at Cambridge, having taken up the piano at six when his mother decided to relearn. Blythe says he was "pedantically faithful" to her recordings.

At 38, there's still something studenty about him. After flirting with directing post-university, he veered into sound design, regularly working with Headlong and the Donmar Warehouse on Enron, Red, for which he won a Tony in 2010, and King Lear, for which he won an Olivier last year.

Does he regret the outcry that initially greeted London Road? "The problem with the word 'musical' is that it's so loaded with implications of dancing, spectacle and fun. Some people, quite rightly, found the notion of making entertainment out of what had happened disgusting," he says. "We didn't want to be seen as doing it for sensational reasons."

London Road focuses on the community rather than the crime. Blythe interviewed a number of Steve Wright's neighbours, ordinary people caught in a media storm. There's a surprisingly uplifting quality to Cork's score. "I wanted every line in every song to have the quality of a memorable tune," he explains. "I wanted it to embed itself in the brain.

"Musical theatre fans are divided. Some of them love it, because it's new and they've not heard anything like it before. Others don't like it at all because it's transgressing a few boundaries. Some fear it will become the norm, which I think is ridiculous."

Next, he's working with Michael Grandage on the soundscape for Peter and Alice, the follow-up to Red, about a meeting between the real-life inspirations for Peter Pan and Alice in Wonderland. Would he do a conventional musical? "Definitely, but I'd always take the lessons I learned from London Road, which is never to apply formulas from the past as an easy method of generating new material. The most exciting stuff always comes from something unexpected."

'London Road', National Theatre, London SE1 (020 7452 3000) 28 July to 6 September