My attention was drawn to an article in the Daily Mail about recent work which suggests that at least part of Offa's Dyke near Chirk can be dated to the period 430-652 with 95% certainty on the basis of radio-carbon dating.
This would suggest that the Dyke was an on-going policy by the Kings of Mercia, and that King Offa (757-796) himself may have consolidated the work of his predecessors into a complete system. The article, with some good illustrations of the earthwork can be read at Ancient Offa's Dyke between England and Wales may be renamed.
Offa's Dyke on Spring Hill in Shropshire
What this does demonstrate is both the ability of Anglo-Saxon rulers to engage in organised projects over along period and their ability to exact labour and military services and, secondly, the fact that we are still learning about the past in ways which open up our understanding.