Complex sentence: The City is a small (one square mile) but influential part of London and the Port of London, within which the duties were payable, stretched far beyond its boundaries all the way along the Thames from Yantlet Creek (downstream from Gravesend) to Staines.
Split into simpler sentences: The City is a small (one square mile) but influential part of London. The Port of London, within which the duties were payable, stretched far beyond the boundaries of the City, all the way along the Thames from Yantlet Creek (downstream from Gravesend) to Staines.