In the history of rock music, German singer and actress Nico has a unique role. She sang with the Velvet Underground, pursued a solo career and has a list of romantic encounters that counts as a "who's who" of music legends. She died 20 years ago but would have been 70 this month. Fellow Velvet Underground member John Cale is organising a concert at the Southbank Centre on Saturday to celebrate her life. He shared his memories of her with reporter Nicola Stanbridge.