Gildas is an interesting example of the differences between studying history in this period and the history of the modern age. In the modern period, only a very small number of highly specialised historians would devote much effort to the examination of a moralistic religious text, whose main purpose was to induce its readers (they would originally have been listeners) to repent their sins. Because Gildas is almost all we've got for mid-sixth century Britain, he is pored over, and every scrap of possible meaning is extracted.

What a historian of this period would say – and what makes the study of such a period not only interesting, but also arguably more demanding but more rewarding than the study of better-evidenced periods – is that even the omissions and unreliabilities of such texts are very revealing. Snyder, in his An Age of Tyrants: Britain and the Britons AD 400-600, points out that histories, intended as such, that mention Britain stop at 429 and don't pick up again until the time of Bede (writing in the early 8th century). But he adds: "What we sacrifice in terms of historical narrative, we make up for by having, for the first time, the voices of the Britons themselves."

We can learn something about 6th century Britain from Gildas's prejudices - the ways, for instance, in which a churchman feels able to criticise secular leaders for their moral failings – and if Gildas doesn't say anything about a topic we'd like to know more about, we ask why it wasn't considered worthy of comment. A good example is his failure to make any mention of Arthur, who - if an historical character at all - ought to have been a prominent British leader of this period; probably this means that, if he existed, he was not at all the towering figure that legend has made him. And study of Gildas's text also reveals much about the social, political and religious climate of the day.

If we look more closely at what Gildas actually left us – a work known as De Excidio Britannæ – we can see it is technically an epistle, comprised of three parts – a "historical" preface and two "complaints," one against British kings and the other against the British clergy. All three, examined in detail, have explicitly moral purposes. The supposedly historical preface, for instance, is carefully constructed to illustrate the "wicked" behaviour of the Britons at the time of Rome's withdrawl. Gildas makes it clear that these "sins" were punished, and hence were responsible for subsequent "calamities" experienced by the Britons.

It's in this context we need to read what Gildas portrays as a foolhardy decision on the part of the Britons to hire Saxon mercenaries to defend their lands against the invading Picts and Scots. And although Gildas was writing in a period of peace, he makes it clear that it was these foolhardy decisions that have determined why, although the Britons have kings (reges), those kings are tyrants (tyrannos).

What all this means is that Gildas is not used by historians today as an uncomplicated guide to the progress of the Saxonisation of England. The series of events that he outlines – a barbarian invasion, the appeal by the Britons to the Romans, who ignore them, and then to the Saxons, who send six ships with mercenaries, with more to follow, the successful collaboration between Saxons and Britons, and then the revolt of the Saxons on the grounds that their rations were not issued, resulting in the ravaging of the whole of Britain to the western sea – none of that is taken at face value now. Rather its narrative is tested against other types of evidence (archaeological, place-name, numismatic and so on). Where it is, it is generally found wanting; most obviously, Gildas makes no mention of the permanent Germanic settlements in the east which we have archaeological evidence for. But Gildas is not ignored. If his narrative is suspect, historians still hope to extract understanding from passing references in his work. Stenton – an old source now (1940s), but still respected – uses the epistle to suggest that

an ordered society existed in southern Britain at that time... that the cities of Britain were not inhabited as before, and that the land was troubled by civil wars between contending kings.