The Facts:

The Ruin of Britain by Gildas

The Gallic Chronicle (anonymous)

The Life of Germanus by Constantius

The letters of Sidonius

The History of the Goths by Jordanes

The History of the Wars by Procopius
 

408 x 410

From hereon Britain is subject to raids by Scots, Picts, and Saxons.
 

The Britons appeal to Aetius, the master of the soldiers in Gaul, for aid against the "barbarians". Whether this is the Picts and Scots (as Gildas says) or the Saxons depends on the placing of the events here. The dates here are the dates that Aetius' was in power. If the "thrice consul" quote by Gildas is taken at face value then a date after 446 would be required.

St. Germanus visits Britain to combat the Pelagian heresy, and leads local forces to defeat raiders, allegedly Picts and Saxons. Apart from this, his visit is unremarkable; Britain would appear to still be part of the Roman world.

The Britons inflict a defeat on the invading barbarians. The earliest possible date for this after the end of effective Roman authority is 410, when it would correspond to Zosimus noting that "the Britons ... armed themselves and ran many risks to ensure their own safety and free their cities from the attacking barbarians". The latest date is shortly after the latest possible date for the appeal to Aetius, which follows from assuming that Gildas has got the Aetius quote correct and in context. In any case, according to Gildas this victory is followed by a period of prosperity, presumably at least a decade or so.

St. Germanus perhaps visits Britain a second time. No war or devastation is reported.

According to the Gallic chronicle, large parts of Britain have fallen into Saxon hands. Whether this designates a particularly large and prolonged raid, a treaty for establishing federate troops, or a military take-over depends upon one's interpretation of other events.

According to Constantius, Britain is a prosperous island.

Following the period of prosperity, there is the threat of renewed Pictish raiding, and then a plague.

Following the plague, a "proud tyrant", probably by the name of Vortigern, is in power as the leader of a council of Britain.  Three keels of Saxons arrive at the invitation of Vortigern, to fight against the Picts and Scots. They are settled in the East of the Island. Their leaders are perhaps Hengest and Horsa (Bede). Reinforcements subsequently arrive.

At the invitation of the Roman Emperor, Riothamus, "king of the Britons", leads a force of considerable size into the heart of Gaul "by way of the Ocean" (Jordanes). It is hard to see how one could infer from this that Riothamus was a king of Brittany, as many authors have. In any case, he confronts the Visigoths in battle but is beaten and retreats into Burgundy.

The Saxon revolt. Britain is plundered, even as far as the west coast. The date is "a long time" after the reinforcement which followed the advent, according to Gildas. Presumably at least 5, probably less than 30 years. Following this, Britons flee into the mountains and forests, or overseas. The upper limit is set by the latest possible date for Gildas' writing of the The Ruin of Britain (see below).

The British rally under the leadership of Ambrosius Aurelianus, challenge the Saxons to battle and defeat them. The date is "a while" (presumably at least a couple of years) after the Saxon revolt. Then follows a period (of considerable time, perhaps 43 years) of fluctuating fortunes in war until Badon.

The siege of mount Badon in which the Saxons are defeated. Gildas is born. Thereafter Britain is partitioned between Saxon and Briton, and there is little fighting between them. But civil wars continue, and cities are largely deserted. Note that we cannot mention Arthur here, as he is not mentioned in any of the contemporary sources. The date for Badon is at least say 5 years, perhaps 43 years after Ambrosius' victory.

Angles, Frisians and Britons have migrated in considerable numbers from Britain to France (Procopius).

Gildas writes The Ruin of Britain bemoaning the morals of his countrymen. This is at least say 27 years and perhaps 43 years after Badon. It is therefore probably at least say 48 (=5+43) years after Ambrosius' victory. This is also implied by the fact that Gildas says that those who witnessed the Saxon revolt and the Ambrosian rally have died and "an age succeeded them which is ignorant".  A date for Gildas' composition of The Ruin of Britain any later than about 575 would be incompatible with the reliably known history of Bede from the beginning of the English conversion (597) on.

The English are in control of most of Southern England.

To illustrate how unconstrained the above chronology is, I have given below some fragments from alternate schemes which manufacture the history in greater detail. They all hinge on (and hence are identified by) the position of Riothamus' doings in Gaul in relation to the Saxon revolt, Ambrosius' victory, and the battle of Badon. Number 5 is closest to the chronology proposed by Dumville, although he has the Saxon advent even later (480x490).
 

1. Riothamus = "Arthur" (successor to Ambrosius)

410        Picts and Scots defeated
410s      Age of abundance
425        Vortigern comes to power
428        Saxons under Hengest are invited (Saxon advent)
430s      Saxons reinforced
445        Saxons revolt
446        Appeal to Aetius for aid against Saxons
450        Ambrosius' victory over Saxons
465        British victory at Badon
469        Riothamus (victor at Badon) leads fatal expedition to Gaul
508        Gildas writes his letter.
 

2. Riothamus = Ambrosius

427        Appeal to Aetius for aid against Picts and Scots
430s      Age of abundance
445        Saxon advent
460        Saxon revolt
465        Ambrosius' victory over Saxons
469        Riothamus = Ambrosius leads fatal expedition to Gaul
 

3. Riothamus = John Reith

450        Saxon advent
463        Saxon revolt
465        British nobles abandon Britain, taking the vanquished army to Brittany
469        Riothamus (Breton leader) is defeated by Visigoths
470        Ambrosius' victory over Saxons
 

4. Riothamus = "Vortimer" (successor to Vortigern)

457        Saxon advent
469        Riothamus (successor to Vortigern) leads fatal expedition to Gaul
471        Saxon revolt
475        Ambrosius' victory over Saxons
 

5. Riothamus = predecessor to Vortigern

446        Appeal to Aetius for aid against Picts and Scots
450        Victory over Picts and Scots
450-70   Age of abundance
469        Riothamus, high-king of the Britons, leads fatal expedition to Gaul
470        Vortigern comes to power
471        Saxons advent
477        Saxons revolt
480        Ambrosius' victory over Saxons
496        British victory at Badon
540        Gildas writes his letter.
 

Some (e.g. John Morris) would exclude many of these scenarios on the basis of the archaeological evidence. In particular, Morris claimed that substantial Saxon settlement must have begun before c.430, and that the battle of Badon must have been towards the end of the 5th century. Most would now see this as misplaced confidence in the dating of saxon cemeteries, and perhaps even misinterpretation of the data (as far as the abandonment of midland settlement in the early 6th century is concerned). Despite these misgivings, one must conclude that any scenario (as for example, that by Dumville) which has the first settlement of Saxons in Britain in the second half of the 5th century would want to have a great deal going for it from the textual evidence in order to be accepted in the face of the archaeological record. This still leaves a great deal of leeway to historians, and story-tellers, and the story told by the later primary sources (as reconstructed by me) fits relatively easily within the constraints applied by what we know to be genuine history.