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Amy Macdonald is late. Four nights into her sold-out British tour, the 20-year-old Scot is showering in the venue she and her band will play tonight. The luridly bling pink-leather-lined tour bus she is crisscrossing the country in may run to many luxuries, but washing facilities are not among them. A further wait occurs as the singer makes a phone call to arrange insurance for the sports car she has just bought. With 500,000 album sales and the not unreasonable expectation of half a million more, she can afford both the wheels and a little leeway with punctuality. For Macdonald is, indisputably, on her way to becoming a star.

Dig a little deeper, though, and any suggestions of diva-like behaviour that such shopping and time-keeping habits imply are quickly dispelled. The shower facilities were, she says, with no hint of complaint, “absolutely minging. But I just got on with it; I’ve used worse”. And, judging by how long the phone call took, Macdonald found herself being treated like any other young driver attempting to secure the necessary cover for getting behind the wheel of a high-performance car – with suspicion.

The shrug with which she greets such setbacks is the same one that accompanies her response to the criticism some have made of her music. Nonbelievers decry the folksy, adult-oriented rock of her debut album, This Is the Life, and question how a musician who was in her teens when she wrote it could end up sounding like that. Statements today such as “There is no set place where my music belongs; it just happily floats through everything, really” are unlikely to appease them. But then Macdonald would probably question her need to address their concerns. She has felt, she stresses, far more patronised by reviewers than she ever did by the people twice her age with whom, as a 15-year-old, she used to share the stage at open-mic nights in Glasgow. “Nobody went, ‘You’ll be much better when you’re our age’,” she recalls, a noticeable edge to her voice.

For the most part, her responses are as measured and matter-of-fact as her music, though she has been known to bare her teeth on occasion. When the X Factor winner Leon Jackson failed to appear at a Hogmanay concert last December, citing tonsillitis, Macdonald, who was suffering from the same illness, but performed at a similar event in Glasgow, accused him of having “lazy-itis”. And, when I remind her of one writer’s comment that she appealed to the “supermarket demographic”, the singer zaps back with the magnificently crisp: “Tesco is the biggest music retailer in the UK, so if you weren’t appealing to the supermarket demographic, then you might be in some trouble.”

This Is the Life is, commercially speaking, the sort of album record labels dream of. With relatively scant promotion, it has been a fixture in the charts since its release last July, and proved its staying power by ascending to the top spot last month. “There was never any of that hype,” the singer says, “none of the ‘This is the next big thing, the album you must listen to’. It was more word-of-mouth and natural than that. And, as a music fan myself, I know what it can be like. I hate being dictated to.”

The key to understanding the album’s success lies, I think, in concentrating on what Macdonald excels at. And, as her recorded output and her performance to a packed hall later that evening demonstrate, her chief talent is for communication. Her huge singing voice, her melodies and her lyrics resonate with people in a way that might be uncomplicated, but gets the message across with precision. And that message is sometimes subtler and more ambiguous than Macdonald’s easy-on-the-ear compositions might suggest. Youth of Today, which seems on first acquaintance off-puttingly arch, reveals, on closer inspection – “You’re just some incapable figure/Thinking you’re bigger than me” – a sharp tongue behind its surface smile.

The song Poison Prince, meanwhile, casts a critical eye over the drug problems of one of Macdonald’s early idols, Pete Doherty. The two have since met, though they have yet to discuss the song. That may have been down to a simple mishearing. “He was being interviewed on television,” Macdonald recalls, “and the guy went, ‘So you’ve met Amy, she’s a big fan. And you’ve heard the song?’ And Pete went, ‘The song?’ ‘Yeah, the song she wrote about you, Poison Prince’. And Pete suddenly glazed over. The guy went, ‘What is it?’ And Pete said, ‘Why would I want to listen to a song about me that is called Poison Prick?’”

If Macdonald’s strengths as a communicator and entertainer seem like old-fashioned virtues beside the sheepish, surly mien of many of today’s copycat indie performers, perhaps that, too, says something about her popularity. She may never be cool, but does cool actually sell that many records? It may help, of course, that she is signed to Vertigo, a label that could also be described as old-fashioned. A common criticism of major labels is that they play it safe and short-term. Vertigo, which is home to guaranteed bankers such as the Killers and Razorlight, as well as a notably varied slew of new signings (see box, right, for a round-up of this young talent), displays a diversity that, if viewed with optimism rather than the customary cynicism, seems genuinely open-minded. A teenage singer with no experience, and a set of songs that bear no relation whatsoever to contemporary music fashions, comes a-calling, and you give her a deal, then help to steer her debut album to No 1. Could that be called untypically imaginative?

It doesn’t hurt that Macdonald is the level-headed individual she is. No hidden heroin habit here. Royalties have been invested in a semi in the Glasgow suburb she grew up in. She keeps a close eye on her business affairs. She happily signs autographs for people who stop her in the street. She gets crowds of all-ages gig-goers dancing along to her songs and leaving with a smile on their faces.

“When you break it all down,” Macdonald says, “it’s a job” – and she means that in a way that isn’t as grimly calculating as it might sound. “If you constantly worry about chart positions and all that other rubbish, it will just wear you down. People are always going to say bad things, and there are days when you think, ‘I cannot be bothered with this’ – but then you go on stage and sing, and you think, ‘This is why I do it. I don’t mind about manky showers, because this is what I love about what I do: the opportunity to perform to lots of people and have them sing your songs back at you.’”

In Sheffield that night, her fans do exactly that. So, will she “do much better” when she’s older? She shrugs again. It’s a gesture that says: “Well, I’m doing pretty well now, aren’t I?”

Amy Macdonald’s new single, Run, is released on March 3; her British tour continues tomorrow in Brighton

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