Complex sentence: The sword that gave the name to the type was dredged from the Thames near Fulham, and must therefore date to after the Roman occupation of Britain began--after the invasion of Aulus Plautius in 43 AD.
Split into simpler sentences: The sword that gave the name to the type was dredged from the Thames near Fulham and must therefore date to a time after the Roman occupation of Britain began. That would have been after the invasion of Aulus Plautius in 43 AD.