Complex sentence: Featherstone has concluded that the original material, dating from late 7th century Mercia, was then used to be included in a late 9th century document and asserts that the Mercian kingdom "was at the centre of the world mapped out by the Tribal Hidage".
Split into simpler sentences: Featherstone has concluded that the original material, dating from late 7th century Mercia, was then used to be included in a late 9th century document. Featherstone asserts that the Mercian kingdom "was at the centre of the world mapped out by the Tribal Hidage".