By Neil Smith BBC News entertainment reporter

Scottish singer-songwriter Tunstall is currently touring the UK

If you have never seen Scottish singer-songwriter KT Tunstall perform live, the effect can be a disconcerting one.

In the flesh she looks like a slightly elfin busker, carrying a guitar that looks a size too big and she has a shy, self-conscious demeanour.

But by using an effects pedal to create a multi-layered track featuring vocals, yelps and percussion, she transforms into a one-woman army, full to the brim with rootsy swagger.

Having won legions of fans in concert venues up and down the country, this raw and energising talent looked cowed by the "posh" surroundings of London's Open Air Theatre.

"Maybe we should hand out blankets, like they do in business class," she joked.

But she swiftly made the place her own, daring the glowering skies to rain on her feisty, soulful parade.

Throaty gusto

Admittedly, it would help if she were able to remember her own lyrics instead of collapsing into embarrassed giggles as she did during False Alarm.

And there were times her twee between-songs banter made you wish your seat had a fast-forward button tucked into the armrest.

Her Eyes to the Telescope album is nominated for the Mercury Prize

But artful, heartfelt ballads like Under the Weather and Heal Over cast a warm glow not a million miles from the Shakespearean comedies that are more often associated with this locale.

And the throaty gusto with which she performed rockier numbers like Another Place to Fall and Miniature Disasters suggests Kaiser Chiefs have some stiff competition for that prestigious Mercury Prize.

A young audience enjoyed every moment - a good thing since KT was on and off the stage in barely an hour.

That said, they did need encouragement to stand up for her next single Suddenly I See - performed, as Tunstall proudly told us, on Top of the Pops earlier that evening.

Her debut release Black Horse and the Cherry Tree remains her strongest tune - which made its placement in the middle of her set rather than at the end hard to fathom.