4th century Saxons on Hadrians Wall?

[Caballo]

Just read Guy Halsall's new book (Worlds of Arthur-Fact and Fiction of the Dark Ages) and one theory that he invites people to look at is that Saxons were settled on the Wall in the late 4th century, and that this is linked to the lack of late Roman army belt sets the.
What evidence for Saxons is there?

From "Finds on the Frontier" I can find some late Roman crossbow buckles on the Wall, and some 6th century Saxon brooches ( eg at Birdoswald). Is there other evidence?