What was the old English heroic poem surviving in a single copy dated around year 1000 featuring its eponymous 6th century warrior from greatland Sweden?

The old English heroic poem you're referring to is Beowulf.

While the poem is indeed dated around year 1000, it's important to note that Beowulf is not from Greatland Sweden.

Beowulf is a legendary Geatish hero (Geatland being a region in southern Sweden) who travels to the land of the Danes to defeat the monster Grendel. The poem is thus named after its protagonist.

It is the only surviving manuscript of an Old English heroic poem, making it a significant piece of English literature and a valuable source of information about Anglo-Saxon culture.