Jamjar,
I tried, along with the Yanks, to make a connection to the Rosebank Kerr's, but failed. My earliest ancestor I can find was James Ker from Kirkbean, in Kirkcudbrightshire in 1771. as you see the spelling at that time was Ker, half his family were registered Ker, and the rest were Kerr, I guess that was down to the person writing the record, but thereafter they were all Kerr. They moved from Kirkbean to Maxwelltown then Dumfries Town, where they remained up to my Grandfathers time.
If we could get back far enough, we would all be the same family though, from the Borders.
James.