The earliest known continental crossings into Scotland/ North Britain were done about 9 or 10 thousand years before. The Hamburg-ish People crossed from Germany and Denmark dryshod (might have been a little marshy in places). This wouldn't have been a one off, it would have been a seasonal, semi-permanent state of affairs. As the available routes across what is now the North Sea changed so the patterns of seasonal migration and settlent would have changed.
Late Upper Paleolithic Hamburg People tanged tools found here in Biggar at Howburn Farm. 14, 000 years old.
http://www.biggararchaeology.org.uk/news03_109_howburn.shtml
This Secrets Of Orkney programme seems to suggest a sudden wave of boats full of voles and stone builders setting off from Rotterdam on the high tide around 3,500 BCE.
The Ness is a very interesting site (I love what appears to be domestic RA), but the whole Orkney-centric "turn-everything-on-its-head" seems a little over-egged perhaps.