For info, I've already contacted roofbox.co.uk and had a really helpful reply from them, including this nugget: apparently Tesla changed the shape of the roof bar fitting points in ~June 2015, and the existing Whispbar roof bars currently won't fit on any newly manufactured Model S. Whispbar are working on it, but as it stands right now, an owner of a brand new Model S can't get Whispbar roof bars.