Semona and Kritenya are fictional languages spoken by fictional characters. To actually reconstruct the language of the builders of Stonehenge is probably impossible. Certainly I would not attempt it. I do have to base the languages on something, however, in order to make them believable. Simply written Jibberish would not be.
The protagonist, Amzai speaks a language that we hardly hear, since the book is entirely narrated from his viewpoint, and the book is, of course, written in English. The names and greetings are based on Basque. Why Basque? It is one of the few pre-Indo-European languages that still exists (Margaret Elphinstone also uses it for her characters in The Gathering Night, and I had to change all my names when her book appeared, so that they didn’t all overlap).
Kritenya is based loosely on Proto-Indo-European, a language ancestral to most European and Indian languages. Archaeologists and philologists cannot agree on when this language group spread into this part of the world, but those who follow the debates will be unsurprised that I (an archaeologist, trained at Cambridge under Professor Colin Renfrew), place it at the time and place I do.
Semona is based loosely on Proto-Celtic, the language ancestral to Welsh, Gaelic, Breton and Cornish. Again, archaeologists and philologists cannot agree on the date at which they spread and frankly, your guess is probably as good as mine.
I make no claim that these were the languages actually spoken, but I do want to give an impression of what it might have been like to arrive in a society where different ethnic groups were mingling for the first time, speaking different languages, in order to explore some of the tensions that must have been involved. I didn’t feel the need to develop a complex “conlang,” like Tolkien’s Elvish or Star-Treck’s Clingon, only to give a flavour of linguistic diversity.