Complex sentence: Cereal grains found in the scoop have been dated to the 12th century BC--the Middle Bronze Age--the earliest date for that tradition, Though Bronze objects have been found that are around a millennium older, the building is the earliest metalworking site in Britain.
Split into simpler sentences: Cereal grains found in the scoop have been dated to the 12th century BC, the earliest date obtained for that tradition and which would be regarded as Middle Bronze Age. Although Bronze objects have been found which are around a millennium older the building is the earliest known for metalworking in Britain.