No, it's not. Yet there are no written sources from the place until Gildas, and Gildas is a nonhistorical polemic; The Anglo-Saxon Chronicle, "Stephanus'" Life of Wilfrid, and Bede's Ecclesiastical History give us the first real post-Roman written historical sources.
There are, of course, other written documents. For instance, there is the legislation of Ine of Wessex, a "king" who controlled a rather embarrassingly small plot of land and who allegedly promulgated a code of laws that were apparently wishful thinking that he had no possible way of enforcing; they are theoretically a seventh-century document included in the law code of Alfred, although technically Alfred could have just been attributing his own laws to Ine to give them a bit of historical cachet. And there are a fair few epic poems, like Y Gododdin, about a cattle raid gone horribly wrong in what is now northern England and which frankly is best read by the casual observer as satire because at least you'll laugh a few times. But none of them give any indication as to events.
Material culture, of course, does not differentiate between "Saxon" and "Briton" and "Roman" and so on (and that assumes that one thinks that they are different things); even if it could, it could not be relied upon to give even a passable timeline for 'invasion'.