Though not nearly as well-known as the ''London Trios,'' these six delightful divertimentos are Haydn’s first pieces of chamber music to expressly call for the flute. Straightforward and full of Haydn’s esprit, they are a treasure trove for all lovers of music-making in the home. The stylistic variety of these cheerful pieces d’occasion surely reflects the fact that they are partly based on earlier models: Haydn’s baryton trios as well as arias and other items from his opera ''Il mondo della lun