Complex sentence: The Metropolitan Borough of St Marylebone was a metropolitan borough of the County of London from 1900 until in 1965 it was amalgamated with the Metropolitan Borough of Paddington and the Metropolitan Borough of Westminster to form the City of Westminster.
Split into simpler sentences: The Metropolitan Borough of St Marylebone was a metropolitan borough of the County of London from 1900 to 1965. In 1965 it was abolished and its former area was amalgamated with that of the Metropolitan Borough of Paddington and the Metropolitan Borough of Westminster to form the City of Westminster.