23-Great Grandfather; P.M. David Cameron's 23-Great Grandfather; and Louis XVII's 20-Great Grandfather. Alexander Cockburn, grandson of William de Veteri Ponte, was appointed "Keeper of the Great Seal " by Robert III of Scotland , some time before 1386. William "of Langton and That Ilk" Cockburn , born about 1465 in Langton, Berwickshire,