The small white building is London's only back garden theatre , The Mermaid . Founded by Bernard Miles and Josephine Wilson , it stands beside Duff House in St Johns Wood . Duff House was formerly St John's Wood School , and it was the school hall , situated beside it , that was transformed into the Mermaid , a private theatre with an Elizabethan style stage .Chairman of the Mermaid until his death was Ivor Novello . September 1952