Complex sentence: After Culloden a few MacTavish families began to Anglicize their family name as Thomson or Thompson--a family name already well established in the border Marches by familes unrelated to the MacTavishes.
Split into simpler sentences: After Culloden a few MacTavish families began to Anglicize their family name as Thomson or Thompson. This was a name already well established in the border Marches, where it was worn by families unrelated to the MacTavishes.