This was originally a reply to an archived message so sorry if it's a bit out of context. I spent a bit typing it on my phone so when it gave me a reply error I was kinda angry.
I haven't studied this period to deeply and it has been a while since I read the books, but I was under the impression that it was more or less a counter point to the written record. Or maybe what someone would have learned in the British educational system. Much like how many things learned in school have been overturned by archiologey/info access. I always laughed when someone who has his name praised in the account or has written an account is described by Uthred as a scandalous rat like character or a complete lier.
I have not looked but is Uthred historical figure? Back when I read the first 6 books I assumed he was made up since he somehow manages to make it to every major battle on both sides so he can be the unsung hero. He also seems to stay in that zone where nobody important likes him but they all want him on their side. In a way Cornwell must be pointing out the details/key figures lost to time.
Uhtred is partially based on a combination of Uhtred the Bold - who was the ealdorman of Northumbria from 1006 to 1016 - and the 10th century Uhtred who was an ealdorman in Derbyshire.
The thing to keep in mind is that Cornwell is writing fiction. Particularly good historical fiction, in my opinion, but fiction nonetheless.
I find that it is so good because he follows Mark Twain's advice of "get your facts first, and then you can distort them as much as you please." Cornwell does a tremendous amount of research for his topic, makes sure he has all the facts available, then uses them, or ignores them, as it suits the story. That way he has a story with a firm foundation in history but with the flow and drama of good fiction. Plus, he tends to explain most of his intentional inaccuracies in his writer's notes, which I like.