Complex sentence: The main focus of the museum is an 18th century yacht, the Peggy housed in the boat cellar, where she has been since the mid 19th century and had been bricked up and forgotten before being rediscovered by workmen.
Split into simpler sentences: The main focus of the museum is an 18th century yacht, the Peggy, housed in the boat cellar, where she has been since the mid 19th century. She had been bricked up and forgotten before being rediscovered by workmen.