Complex sentence: The justices of the City and Liberty of Westminster took it over and an octagonal guildhall with a Doric portico was constructed by Samuel Pepys Cockerell in 1805.
Split into simpler sentences: The justices of the City and Liberty of Westminster took it over. They commissioned a guildhall, designed as an octagon with a Doric portico by Samuel Pepys Cockerell in 1805.