Our Best British Female didn't need to convince her sold-out crowd that she was worthy of her Brit Award. There was more teenage screaming here than at a boy band concert.

At Kate Nash's third performance since her triumph over Leona Lewis, KT Tunstall and others, she still seemed shy and was often inaudible between songs, but as she bashed at her keyboard during the evening's more raucous moments, she had the confidence of someone who knows her music has found favour with thousands.

Still only 20, the Harrow resident writes songs from the perspective of a teenage girl, with lines about wearing black and a boy being sick on her new trainers.

At their best they offer the thrill of reading someone else's diary, but followed too often by the realisation that this person doesn't have much of interest to say.

Mouthwash was a fine tune with lyrics that revelled in the mundane, offering the revelation: "I use mouthwash/Sometimes I floss/I've got a family/And I drink cups of tea".

Her proud ordinariness has attracted a fanbase that finds it easy to identify with her, one so young that she had to introduce special guest Billy Bragg twice to raise the desired level of enthusiasm. They duetted on his song New England and he was off again in a flash, leaving Nash looking sweetly starstruck.

Backed by a restrained all-male band, the dominant sound was her plinky-plonk piano, which she pounded with such enthusiasm that on tracks such as Mariella she resembled Victoria Wood bouncing through another comedy ditty.

When the song climaxed and she began screeching: "Never ever ever ever" ad infinitum, she sounded like nothing more sophisticated than a six-year-old discussing the possibility of sprouts.

She was more subtle when she switched to acoustic guitar for ballads such as Birds and The Nicest Thing ("This is my quiet one, so shut up, yeah?").

Her hit Foundations prompted the largest singalong. This is the song so surprisingly successful last July that her debut album had to be rush released in its wake.

It successfully balanced its quirkiness with genuine emotion, something Nash does not always manage. Her Brit is quite some vindication, but she will need to tread carefully now if she is to avoid novelty status.