Dr. Oppenheimer has relied on work by Peter Forster, a geneticist at Anglia Ruskin University, to argue that Celtic is a much more ancient language than supposed, and that Celtic speakers could have brought knowledge of agriculture to Ireland, where it first appeared. He also adopts Dr. Forster’s argument, based on a statistical analysis of vocabulary, that English is an ancient, fourth branch of the Germanic language tree, and was spoken in England before the Roman invasion.

English is usually assumed to have developed in England, from the language of the Angles and Saxons, about 1,500 years ago. But Dr. Forster argues that the Angles and the Saxons were both really Viking peoples who began raiding Britain ahead of the accepted historical schedule. They did not bring their language to England because English, in his view, was already spoken there, probably introduced before the arrival of the Romans by tribes such as the Belgae, whom Julius Caesar describes as being present on both sides of the Channel.

That has not stopped the attempt. Stephen Oppenheimer, a medical geneticist at the University of Oxford, says the historians’ account is wrong in almost every detail. In Dr. Oppenheimer’s reconstruction of events, the principal ancestors of today’s British and Irish populations arrived from Spain about 16,000 years ago, speaking a language related to Basque.

More information on Dr Oppenheimer: Wikipedia article and another article in Prospect. I suppose I'll have to get his book.