Complex sentence: The property has been open to the public since 1951 and is operated by the National Trust with the assistance of the city of Plymouth--the City of Plymouth Museums and Art Museum use the building to house part of their collection.
Split into simpler sentences: The property has been open to the public since 1951. The National Trust operates it with the assistance of the city of Plymouth--the City of Plymouth Museums and Art Museum use the building to house part of their collection.