Hi Dave,
One thing appears certain, by the C5 AD the tribes to the north of the Wall were affiliated with the Romanised population to the South; struggling through Gildas there are strong hints that Ambrosius's power base was the non-conquered peoples around the Wall and that, with the Roman withdrawl, it was these tribes, either in alliance or in some sort of tributary arrangement, that were enforcing the line against the Picts. I suspect that this is the origin of the later divide of "Scotland" into the Pictish Kingdoms and the Kingdom of Strathclyde.
What is very evident is that, although Hadrian drew the line of the frontier, Roman influence and lifestyle permeated a lot further north - probably because of the economic effect of the Wall garrison and that these people, 300 years later, felt that their interests lay with the Romans to the South rather than the Picts to the North.
Pete
God is our guide! from field, from wave, From plough, from anvil, and from loom; We come, our country's rights to save, And speak a tyrant faction's doom: We raise the watch-word liberty; We will, we will,we will be free!