Complex sentence: Leigh Woods has been built on since 1865 and the land south of Nightingale Valley was fully developed by 1909--the rest has been preserved by gifts of land by the Wills family, and is now owned by the National Trust.
Split into simpler sentences: Leigh Woods has been built on since 1865, and the land south of Nightingale Valley was fully developed by 1909. The rest has been preserved by gifts of land by the Wills family and is now owned by the National Trust.