Gildas

Gildas (c. 500 – 570) was a 6th-century cleric of north Welsh origin. He is one of the best-documented figures of the Christian church in the British Isles during this period.  His work De Excidio et Conquestu Britanniae, which contains narratives of the post-Roman history of Britain, is the only substantial source for British history of the fifth century [1], although written one century after the facts. He was ordained in the Church, and in his works he favours the monastic ideal.

De Excidio et Conquestu Britanniae

Gildas' principal work, De Excidio et Conquestu Britanniae, is a sermon in three parts condemning the acts of his contemporaries, both secular and religious. The first part consists of Gildas' explanation for his work and a brief narrative of Roman Britain from its conquest under the principate up to Gildas' time. He describes the doings of the Romans and the groans of the Britons, in which the Britons make one last request for military aid from the departed Roman military. He excoriates his fellow Britons for their sins, while at the same time lauding heroes such as Aurelius Ambrosius, whom he is the first to describe as a leader of the resistance to the Saxons. [2]
Part two consists of a condemnation of five British kings, Constantine, Aurelius Conanus, Vortiporius, Cuneglas, and Maelgwn. As it is the only contemporary information about them, it is of particular interest to scholars of British history. Part three is a similar attack on the clergy of the time.
Sadly, Gildas never had the intention to write history, so objectivity was not of his concern. 'De Excidio' is a sermon, a preach in which Gildas simply uses history as an illustration. Dates were unnecessary, so Gildas gave none. When one reads his description of the Anglo-Saxons, then it is very clear that Gildas absolutely hated those people. We do not know the reason why. Despite this hate, Gildas never mentioned a language imposition by the Anglo-Saxons.

Bede

Bede (672 / 673 – 26 May 735 AD), also referred to as Saint Bede or the Venerable Bede (Latin: Beda Venerabilis), was a monk at the Northumbrian monastery of Saint Peter at Monkwearmouth, today part of Sunderland, northeast England, and of its companion monastery, Saint Paul's, in modern Jarrow (see Wearmouth-Jarrow), both in the Kingdom of Northumbria. Bede was a proto-English speaking person. He mentions the existence of English in Britain. Like Gildas, Bede never mentioned a language transition in Britain.

He lived about one hundred years after Gildas of whom he used a maximum of text, although correcting Gildas sometimes where he could.
Bede wrote scientific, historical and theological works, reflecting the range of his writings from music and metrics to exegetical Scripture commentaries. He knew patristic literature, as well as Pliny the Elder, Virgil, Lucretius, Ovid, Horace and other classical writers. He knew some Greek and Hebrew. His Latin is generally clear, but his Biblical commentaries are more technical.

Bede's best-known work is the Historia ecclesiastica gentis Anglorum, or An Ecclesiastical History of the English People. Completed in about 731, Bede was aided in writing this book by Albinus, abbot of St Augustine's Abbey, Canterbury. The first of the five books begins with some geographical background, and then sketches the history of England, beginning with Caesar's invasion in 55 BC. A brief account of Christianity in Roman Britain, including the martyrdom of St Alban, is followed by the story of Augustine's mission to England in 597, which brought Christianity to the Anglo-Saxons. An other five books relate the story of Christianity and its relation with worldly power up to Bede's time. Unlike Gildas, Bede tries to give good dates for the events he wrote about.

Nennius

Nennius was a Welsh monk of the 9th century. He has traditionally been attributed with the authorship of the Historia Brittonum, based on the prologue affixed to that work, This attribution is widely considered a secondary (10th century) tradition.
Nennius was a student of Elvodugus, commonly identified with the bishop Elfodd who convinced British ecclesiastics to accept the Continental dating for Easter, and who died in 809 according to the Annales Cambriae.

Nennius is believed to have lived in the area made up by present day Brecknockshire and Radnorshire counties in Powys, Wales. He lived outside the Anglo-Saxon kingdoms, isolated by mountains in a rural society. Because of the lack of evidence concerning the life of Nennius he has become the subject of legend himself. Welsh traditions include Nennius with Elbodug and others said to have escaped the massacre of Welsh monks by Ethelfrid in 613 by fleeing to Scotland.

The Historia Brittonum, or The History of the Britons, is an attempt of a historical work that was first composed around 830, and exists in several recensions of varying difference. It wants to tell the history of the Brittonic inhabitants of Britain from earliest times, and this text has been used to write a history of both Wales and England, as it was one of the more reliable sources. Nennius is traditionally named as the author of the text, though this is widely considered a secondary tradition, originating in the 10th century. Bede's work is considered to be more serious than Nennius' texts. Nevertheless, Bede gave us 446 AD as the date of the coming of the Anglo-Saxons, Nennius mentioned 428 AD.  This website explains why 428 is far more likely.

Anglo-Saxon Chronicle & other sources

The AS-chronicles were probably written one hundred year after Nennius. They are rather unreliable. But as they again give some extra details of what happened during the fifth century, they cannot be dismissed entirely.
More authors wrote during the early Middle Ages, such as Geoffrey of Monmouth, but here we enter the realm of myths and legends, like the legend of King Arthur.