There are up to 40 photographers in the media room but very little for them to photograph. Only three of the nominees for this year’s Mercury Music Prize have turned up. British Sea Power and the Portico Quartet are smiling for the cameras. But, having played Ghosts – the opening song from her shortlisted album Alas I Cannot Swim – Laura Marling has gone. Caught somewhere between amusement and exasperation, her publicist explains that Marling is on the fourth-floor roof terrace with no intention of coming down.

As the lift ascends, you can’t help but imagine the track marks on the floor where an army of PRs and record company people had tried to drag their shy artist towards the photo opp. In truth, Marling is nudging a piece of pitta bread into a mound of humous – a snack to keep her going until a lunch date in an hour. As though it were the most reasonable thing in the world, she restates her position vis-à-vis the photocall: “I’m attempting to avoid it at all costs.”

Everything about Laura Marling today points to an artist, still only 18, resolutely ring-fencing her right to not feel freaked out by her circumstances. Asked about the sort of day she has had so far, she seems happier to discuss the root canal surgery her flatmate is having right about now. “I popped my head through her door this morning, and told her that I loved her very much,” she says, before conceding that, yes, she did also find time to tell her family what she would be doing today. Her father didn’t know what the Mercury was. Not so her sisters – seven and nine years older than her – “who would play me their favourite records in an attempt to make me cool. I remember when PJ Harvey won the award in 2001. She was on the TV and they were really thrilled.”

Talk to Marling long enough and a picture of a rarefied musical upbringing emerges. Her earliest memories are of seeing bits of studio equipment lying around the house. When she was 2 the recording studio her father ran went bankrupt. He had to dismantle it and sell it piecemeal. At weekends, his Joni Mitchell albums were a constant soundtrack. She remembers crying, “aged 5 or 6”, because her father was away and her mother didn’t know where he had put Mitchell’s greatest hits CD. “She had to phone him up because I was inconsolable.” Court & Spark is still her favourite album.

All things considered, it’s no surprise that Marling developed her own talent as quickly as she did. Nevertheless, Alas I Cannot Swim is an exceptional debut by whatever criteria you gauge it; an intricate collage of wisdom reluctantly gained in the wake of what appears to have been a bad relationship or two. “Lover please do not fall to your knees/ It’s not like I believe in everlasting love,” she sings on Ghosts with the air of someone attempting to downscale the agony of separation to mere inconvenience.

Unlike her emerging female solo contemporaries – Duffy and Adele spring to mind – there’s nothing about Marling that would lead you to believe that the desire for stardom has propelled her to this point. You can only wonder what her new paymaster at EMI, Guy Hands, would make of assertions such as, “None of my heroes release singles. I find the way that people are promoted and sold . . . I just . . . it really sickens me.” Clearly, Marling’s timorous exterior conceals a granite-hard resolve.

Had Marling – the most famous musical alumnus of Leighton Park Quaker school in Reading since the composer Richard Rodney Bennett – stuck around for her A levels she might have contributed to the school’s 97 per cent pass rate. Unfortunately, she hated it there. “I know it sounds like a terrible teenage thing to say, but I never enjoyed it,” she says. “I mean, I was fascinated by everything I was learning. But there was this fear of standing out from the crowd. In Reading that isn’t a good fear to have. Besides, I prefer to find out things on my own. I always have done.”

So off she went, aged 16, to London, only to find that breaking away merely made her “paranoid that I was less of a person for not finishing school”. Given that adolescence is founded on the fear that you have either made or are about to make a terrible mistake, it’s no surprise that she was paranoid. What undoubtedly helped her to adjust was that she found a peer group that helped to neutralise her social inadequacies. After attending her show – only her second – two years ago, the like-minded pop bard Jamie T asked her to support him on tour. Until the beginning of this year she also played with fellow Reading indie gentlefolk Noah and the Whale. That she can no longer make time to play with them is, she says, “heartbreaking”.

Their association continues, however. The breathtaking chamber-folk arrangements on Alas I Cannot Swim were dreamt up by the group’s frontman Charlie Fink. His encouragement seems to have been central to her development. When Marling struggled to deliver a satisfactory vocal onYour Only Doll (Dora), she left to have a cigarette only to come back and find that Fink had dimmed the lights and encircled her microphone with potplants and tea-lights.

The first time she heard the album in its entirety was near the end of her time with Noah and the Whale. “We listened to it on the way from Dublin to Belfast and the length of the album was the exact journey time. How did I feel? Very, very emotional. We were all starting to get successful in our own bands . . .”

Her voice trails off. What surely ought to be a happy anecdote assumes a poignancy that has everything to do with the fact that this part of her life has now ended. If Marling had a band with which to share the impact of this sudden attention you suspect she may well be downstairs braving the flashbulbs. Right now, though, on the fourth floor, fame feels far more manageable.

Isn’t there even a tiny part of her that wants to go down and see what it’s like so she can tell her friends about it afterwards? She smiles. “In other words, how much am I willing to suffer for a good anecdote? Hmm . . . I think I’m leaning towards keeping my sanity.”

One more try. I remind her that this might never happen again, and this time she nods and lets slip a quiet, “OK then.” But when the time comes she calmly strides past the hubbub, unnoticed, just in time for her lunch date.

Alas I Cannot Swim is released by EMI

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