JFW: Robin Hood ballads go back to 600 AD, when he would have spoke a language similar to modern Icelandic.

Interested to see your sources for this as, AFAIK, there are no ballads about him recorded before the early 14th century. Since ALL the early stories and ballads are set in post-conquest times, it's hard to see where a pre-conquest date would fit the legends. Sure you're not confusing him with Arthur?