SION HOUSE, ISLEWORTH.
Sion House, which was a nunnery four centuries ago, occupies an imposing position on the Middlesex bank of the Thames, nearly opposite the western extremity of New Gardens. It is now one of the residences of the Duke of Northumberland, who generally prefers his house at Albury, in Surrey, when staving near London. Within Sion House are many of the invaluable treasures from Northumberland House, which was pulled down when Charing Cross was altered, but these cannot be inspected by the public. The famous lion, taken from the crest of the Percy family, that stood for 125 years over the great entrance to Northumberland House, was removed to Isleworth in 1874, and is conspicuously placed on the top of Sion House. The gardens are justly celebrated.
source: The Queen's London : a Pictorial and Descriptive Record of the Streets, Buildings, Parks and Scenery of the Great Metropolis, 1896