Complex sentence: It is the grounds of a grade I listed, neoclassical 18th century country house called Heaton Hall remodelled to a design by James Wyatt in 1772, which still stands and is open to the public as a museum and events venue.
Split into simpler sentences: It is the grounds of Heaton Hall, a Grade I listed, neoclassical 18th century country house. The hall was remodelled to a design by James Wyatt in 1772, and is now open to the public as a museum and events venue.