Complex sentence: Colchester claims to be Britain's oldest "recorded" town, on the basis that it is mentioned in passing by Pliny the Elder, the Roman writer, in his "Natural History" ("Historia Naturalis") in AD 77.
Split into simpler sentences: Colchester claims to be Britain's oldest "recorded" town. Its rationale is on the basis that it is mentioned in passing by Pliny the Elder, the Roman writer, in his "Natural History" ("Historia Naturalis") in AD 77.