Upvote:2
Eohric probably didn't have a banner but he probably did have a flag of some type. A banner is a heraldic flag with a coat of arms.
https://www.google.com/search?q=banner+of+the+holy+roman+emperor&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjw74Cj7bbUAhXFbD4KHbYwCoAQ_AUICigB&biw=1280&bih=894#imgrc=eauU78zZuA2YkM:1
https://en.wikipedia.org/wiki/Heraldic_flag#/media/File:Funeral_Elisabeth.jpg1
https://flagspot.net/flags/pl_royal.html2
Since Eohric lived before heraldry he couldn't have had a heraldic banner, so it would be more correct and less anachronistic to speak about his flag or flags.
A shield of three golden crowns, placed two above one, on a blue background has been used as a symbol of East Anglia for centuries. The coat of arms was ascribed by medieval heralds to the Anglo-Saxon Kingdom of East Anglia and the Wuffingas dynasty which ruled it. The arms are effectively identical to the coat of arms of Sweden.
https://en.wikipedia.org/wiki/East_Anglia3
https://commons.wikimedia.org/wiki/File:Coat_of_arms_of_East_Anglia.svg4
http://www.crwflags.com/fotw/flags/gb-eangl.html5
https://en.wikipedia.org/wiki/Three_Crowns6
https://www.google.com/search?q=coatarms+of+King+Arthur&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjSmY7M8rbUAhXDcz4KHfI0ACQQsAQIXg&biw=1280&bih=894#tbm=isch&q=coat+of+arms+of+King+Arthur7
Since that symbol dates to centuries after Eohric's era novelist Bernard Cornwell probably didn't see any reason to use it. And he probably based the description of Eohrics's flag only on his imagination with no evidence.
[1]: https://www.google.com/search?q=banner%20of%20the%20holy%20roman%20emperor&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjw74Cj7bbUAhXFbD4KHbYwCoAQ_AUICigB&biw=1280&bih=894#imgrc=eauU78zZuA2YkM: