In our age of X Factor celebrity, it's easy to forget what a big deal Lena Zavaroni was. Back in the 1970s, this child star was a household name; not just in her native Scotland, but around the world. At the age of ten, the Rothesay girl became a fixture on Opportunity Knocks, and with a voice trained by singing in the family chippie, she won the TV talent contest a record five times. It was only to give other acts a chance that she stepped aside.

Belting out 'Ma! (He's Making Eyes At Me)' with the gusty swagger of someone several times her age, Zavaroni performed for President Gerald Ford and the Queen, and shared a bill with Liza Minnelli and Shirley Bassey. She had several TV series in her own name, often appearing in the company of fellow child star Bonnie Langford. It seems odd that the youngest performer ever to have a top ten album should have slipped from public consciousness, but if you remember her, it's probably a clue to your age. Certainly for 26-year-old Erin Armstrong, Zavaroni meant nothing before she landed the lead role in Lena, a new musical about the star's life.

'I'd never heard of her, which is insane given how talented and well-known she was,' says Armstrong. 'My parents' generation are like "yeah! How can you not know who she is?" But not many people my age have heard of her.' Having got up to speed, she's determined to change that. 'I want to do her justice,' she says. 'She means a lot to me now and there's a lot to her story that people don't know about.'

Armstrong is best known to fans of Shetland as Cassie, stepdaughter of Douglas Henshall's detective Jimmy Perez. Recently home from filming the next series, she's thrilled to be making her professional stage debut not only as an actor but also a singer. 'They're big boots to fill,' she laughs. 'She had an incredible voice in her 30s, but when she was discovered she was only ten and this voice was coming out of her that sounded so mature. I've always loved singing, so it's amazing to be offered the role.'

Armstrong is relishing the chance to learn from her fellow cast members, including Dead Ringers actor and impressionist Jon Culshaw in the role of TV host Hughie Green. 'It'll be really interesting to see how he approaches playing Hughie Green; he is a theatrical character but there are also darker tones there which Jon will capture brilliantly.'

Tim Whitnall's script, she says, celebrates Zavaroni's charm, reveals her off-stage gutsiness, and also deals sensitively with her history of mental illness. The star's death from pneumonia at the age of 35 followed a history of anorexia nervosa and depression. 'The script is beautiful, poignant and sad,' Armstrong notes. 'We think it's important to showcase that other side of her. We're telling the truth about what she faced because that wasn't always highlighted throughout her life.'

Lena, Beacon Arts Centre, Greenock, Wednesday 16–Saturday 19 March.