Thought I'd throw in my tuppence worth...
There is little to no evidence of sails in the 'north' before the 7th century even though sails were in evidence around the Med. and further afield in Arabia/China during this period (and obviously much earlier). I don't think it unreasonable to assume that northern Europeans would have seen these in action what with all of the trading that was taking place around this period. That said, Med. trading collapsed in the 700s with the rise of Islam and the disruption that brought to trading routes, but the rise of European emporia succeeded this fall in Med. trading, and was facilitated by shipping. Could this have been the cause of northern ships adopting sails so they could take over the trading functions lost from the Med. thus ensuring the supply o fthose sought after luxury goods?
Early written evidence includes Alfred the Great's writing down of the stories of Othere and Wulfstan in the 870s/80s and a variety of tax codes, legislation and regulations that were written down that included details of shipping. The archaeology for 'northern' boats like Sutton Hoo and Kvalsund suggest they were too round bottomed to be able to tack successfully when under sail and were more suited to movement under oar power. Other evidence for shipping includes; traces of structures, behaviour (dumps) and distribution of goods (Anglo-Saxon coinage). However none of this ephemeral evidence really indicates the presence of sails, only hints at them.
Fascinating subject; thanks for raising it.