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Two of the UKs most successful songwriters are swapping pop music for musicals as they adapt a David Walliams book for the stage.

In 2003, when Little Britain was first televised, thebest-selling book of the yearwas Harry Potter and the Order of the Phoenix.

Its author JK Rowling might not have known it at the time, but one of the BBC comedy shows breakout stars would go on to become one of her peers as a hugely popular childrens writer.

Indeed, David Walliams found himself not justoutselling Rowling in 2017, but also veteran childrens authors like Philip Pullman and Julia Donaldson.

(Admittedly, Rowling hasnt released a new childrens book since Potter – but that series continues to sell by the bucketload.)

Walliamss hugely popular books such as Gangsta Granny and The Worlds Worst Children have become a publishing phenomenon and been translated into 53 languages.

Hes almost like a continuation of Roald Dahl, says Nick Coler, who has adapted Walliamss Billionaire Boy as a new stage musical with songwriting partner Miranda Cooper.

Those stories have always been really popular because theyre a bit cheeky and a bit wrong. And theres nothing like kids for wanting that sort of thing.

In case you dont have Walliams-obsessed children of your own, the billionaire boy at the centre of Billionaire Boy is 12-year-old Joe Spud.

The premise is that Joe has all the money he could possibly want, hence the books title, but no friends – something he is keen to put right.

When it came to bringing the story to the stage, there were fewer people better placed than Cooper and Coler – two of the UKs most respected and prolific songwriters.

If their names sound vaguely familiar, it may be because youve spotted them in the credits of countless pop hits over the last two decades.

As part of songwriting team Xenomania, they were responsible for hits by Sugababes, Kylie Minogue, Alesha Dixon and Sophie Ellis-Bextor.

Perhaps most notably, they were the brains behind just about every major Girls Aloud track – a group loved by critics for songs such as Love Machine and Biology, which took more innovative approach to pop.

In an article for The Guardian, published at the height of their success, Craig McLean said: These are pop songs that dont follow the rules of pop.

Choruses take yonks to come in. Contrary musical ideas are mashed together (anyone for a trumpet break? A jolt of robo-disco?)

The Telegraph,meanwhile, acknowledged Xenomanias role in making the group arguably the most musically inventive British pop act of the decade.

But writing a stage musical is, of course, an entirely different discipline to writing a pop single.

Its far more rigorous lyrically, it obviously has to move the narrative of the show along, whereas a pop song can often be much more static, Cooper explains.

Which presumably means its a case of starting with the lyrics, and then coming up with the melodies?

Yes, which is actually not dissimilar to how I wrote a lot of the Girls Aloud songs, she says.

I would always have a notebook, just lots of concepts or first lines of choruses or just words, just so I was constantly trying to keep myself fresh in terms of what song I was writing, and not going into the rut of writing the same song over and over again.

So with Cant Speak French, I had the lyric I cant speak French, so I let the funky music do the talking, talking. With Gabriella Cilmi, I had Sweet about me, nothing sweet about me.

And the great thing about that is that you just keep singing the same lyric until you get a killer melody.

But Coler points out that writing music for a stage show requires more than just catchy hooks.

In a musical, you cant just have the script going along telling a story and then you have a song – the story has to run through the song to the other side, and move the plot forward.

Whereas a pop song is a very self-contained thing… theres a thin line between making something repetitive but also making it interesting, and doing it all in under three minutes.

Reflecting on her time writing such songs with Xenomania, Cooper adds: We had the luxury of time… so we were able to experiment.

[Girls Alouds] Biology, Love Machine, Sound of the Underground, they all came out of experimentation, which you dont really do if you have one day and the clock ticking, you probably play it safe and say whats in the charts right now, lets do something like that.

Wed start an idea and come back to it a week or month or years later. With Call The Shots we wrote the chorus of that two years before we wrote the rest of it. So things were allowed to distil.

Coler left Xenomania on good terms in 2010. By that point, Girls Aloud had started to wind down, and he was keen to pursue other ventures.

Theres only a five-year period of when you can be successful [as a songwriter] because nobodys caught up with you, and then people start to emulate the sound youre making, he explains.

And obviously as they do that, they start to water down what you sound like. So the process sort of starts to break down in a way, and you have to start again, and I think around that time we were all feeling like we wanted a change anyway, and I had a few other projects.

Coler and Cooper kept in touch, and the first musical they wrote was a stage adaptation of 2007 film Son of Rambow, which debuted earlier this year.

Sam Hodges, the artistic director of the Nuffield Theatre in Southampton, where that show played, then asked the pair if theyd work on another.

Billionaire Boys music, which accompanies the script adapted by Jon Brittain, was written in the space of just a few months – quite a feat considering musicals often take years to put together.

We were fans of musicals, but wed never written one before Rambow, which I think has served us quite well, because we dont know the rules, laughs Cooper.

So weve been breaking them left right and centre! And sort of just going on instinct.

Walliams has had a fairly hands-off approach, Cooper explains, allowing them carte blanche to just get on with it.

Hes been really supportive, she adds, but I guess the punch will come when he sees it!

Billionaire Boy opens at Nuffield Southampton Theatres on 28 November.

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