Well, to narrow it down a bit it'll help to know if it's a successor chapter of one of the bigger Space Marines Chapters or a complete new one.
And also, is there something special about your chapter that repeats itself in its background? Like extreme Bravery or Bloodshed?
I personally wouldn't go with Anglo-Saxon Heraldry, it's just to much work & unless you plan to put that herald on any tank & Vehicle
i'd go with something simple, maybe a Greek letter for it's company insignia.
I draw quite a lot an created my own insignia & Herald for my Chaos Warband, and to figure out what i wanted i simple implemented the models
i personally like the most, the Chaos God i play the most & on what Traitor Chapter this Warband is based on.
If Antique to Medieval Age is what you draw your Inspiration from, i'd go with something like GrimTeef already suggested; Dragons, Lions - maybe
some crossed Weapons like a Crozius and an Power Maul or something like this. If you have a base idea i'd like to help you out - but some base
idea needs to come from you