I wrote to Professor Colin Renfrew at Cambridge University, who is both a top archaeologist and a very proficient linguist. He reckons that yan, tan, tethera is Pre-Celtic, but also Proto-Anatolian-Celtic, i.e. a very early form of Indo-European. It was from an earlier migration (before the historical Celts)so I was partially right, but he put me right about the Hittites, they only date to 1,600 BC. So I think that that is pretty definitive; I certainly don't want to argue with the prof!