Did they really arrive in AD 449?
In fact they arrived much earlier. The late Roman historian Ammianus Marcellinus records the Saxones among the barbarians (along with Picts and Scots) who were harrassing the Britons in about AD 365, and the mid-fifth-century Gallic Chronicle mentions another severe raid in 410, and the fall of Britain to the Saxons "after many troubles" in 441. The date "449" comes at the end of a long history of confusion.
The confusion starts with Gildas in the sixth century, who wrote the first British account of the Anglo-Saxon invasions, without including a single date. He did however mention that just before the British invited the Anglo-Saxons, they sent an appeal to one Agitius, who was three times appointed a Roman consul. This is probably Aetius, whose third consulship began in AD 446.
We move on to Bede in the eighth century, who mentions the Saxon invasion in a couple of places, in his Chronica Maiora of 725 and in the Ecclesiastical History of 731. Bede was one of the early adopters of A.D. dating, but in his Chronica, he was using the then-common method of assigning events not to years but to the reigns of the contemporary Roman emperors. He seems to have decided from the fact that Aetius became thrice-consul in 446 to put the Saxon invasion in the following reign, the joint reign of Martianus and Valentinianus (Valentinian III ruled the Roman Empire in the west from 425 to 455, and Marcian ruled in the east from 450 to 457). When Bede came to include the Saxon invasion in his History, he added an (incorrect) A.D. date, and wrote "In the year of our Lord 449, Martianus, forty-sixth [emperor] from Augustus, took the kingdom with Valentinian, and ruled for seven years. At that time the race of the Angles or Saxons..."
The earlier sections of the Anglo-Saxon Chronicle were composed in the late ninth century, and they simply repeat Bede's words against the year 449. Later writers saw the invasion listed against the year 449, ignored the context, and invented a legend.
Set against these contradictory written records, archaeology promises more impartial results. Unfortunately, none of the remains can be dated with the precision that historians are used to: a recent survey refuses to be any more precise than the half-century. But in these general terms, one can see a few sites with identifiably Anglo-Saxon remains in the first half of the fifth century, but a great increase in the number and density of Anglo-Saxon sites over the second half of the fifth century and the first half of the sixth.
So although 449 was not the date of the first arrival of the Anglo-Saxons, it is perhaps fair to say that by then it was clear that they were here to stay.
[For the archaeology, see J. Hines, "Philology, Archaeology and the adventus Saxonum vel Anglorum", in Britain 400-600: Language and History, edd. A. Bammesberger and A. Wollmann (Heidelberg, 1990), pp.17-36]