A key collaborator in the creation of the Theatre Green Book was the historic East Sussex opera house Glyndebourne, renowned for its summer festival which draws thousands of opera lovers to the stately home's verdant grounds each year. Glyndebourne has been forging a path towards greater sustainability in opera for some time. "Art, opera, nature [has always been] a core trinity for Glyndebourne," explains its archivist Phil Boot in a new BBC documentary.