The bind is in when they arrived in western China and Tibet. One search return states that earliest Tocharian (Sakas) inscriptions date to the 7th century BC. But DNA is a problem; the DNA of the central Asian PIE-speakers contained a marker absent from the central European Halstatt Culture populations. This discussion thread from six years ago may help:
http://listsearches.rootsweb.com/th/read/GENEALOGY-DNA/2004-08/1092252776
Protohistorical accounts transmitted by classical historians and archaeological support suggests the "Celtic migrations" began in the 7th century BC, and that Britain received two waves of Celtic settlement, late 7th and 4th=>1st century BC. (The tribal names were often found on both sides of the Channel: Parisii, Atrebates, Volcae, Belgae, one or two others. The name "Volcae" => A-S "Wealas", "Wealh" => Engl. "Wales", "Welsh". "Volcae" was also the name of a tribe in Haute-Provence, inland from Nice and Monaco. I think the British Volcae lived somewhere around Bath, in western England; their neighbors were the British Belgae, who lived in Venta Belgarum -- Winchester -- over west to Salisbury and Bath.)
The Celts also had substantial migrations to the south in the 7th down to the 3rd centuries BC, crossing the Alps to descend upon and destroy the Etruscan colony city of Melpum in the Po valley in about 609 BC, and sacking Rome in 390 BC. Three Gaulish tribes (the Tectosages, Tolistobogii, and Trocmi) overran Greece in the 270's BC and were finally defeated east of the Dardanelles by King Eumenes II of Pergamon, after which he settled them in Galatia, a district around modern Ankara. By Julius Caesar's time, the Galatians were still speaking Gaulish and contributing heavy infantry, armed in the Roman legionary fashion, to King Mithridates. Their language survived into imperial times.
"And what of the Celts we know – the Irish, Scots and Welsh? Scholars have traditionally placed their origins in Iron Age Central Europe, but Oppenheimer’s new data clearly show that the Welsh, Irish and other Atlanticfringe peoples derive from Ice Age refuges in the Basque country and Spain.They came by an Atlantic coastal route many thousands of years ago, though the Celtic languages we know of today were brought in by later migrations, following the same route, during Neolithic times."
"Stephen Oppenheimer shows us, in his meticulous analysis, that there is in truth a deep genetic line dividing the English from the rest of the British people but that, fascinatingly, the roots of that separate identity go back not 1500 years but 6,000. The real story of the British peoples is one of extraordinary continuity and enduring lineage that has survived all onslaughts."
My dad's mother (Mrs. Smith) has the same mtDNA as Cheddar Man