Complex sentence: Although the extent is much reduced due to enclosure of common land from the millions of acres that existed until the 17th century, a considerable amount of common land still exists, particularly in upland areas, and there are over 7,000 registered commons in England alone.
Split into simpler sentences: Although the extent is much reduced due to enclosure of common land from the millions of acres that existed until the 17th century. However, a considerable amount of common land still exists, particularly in upland areas, and there are over 7,000 registered commons in England alone.