Thanks riverman. I've only now had time to study the link you supplied at http://www.shef.ac.uk/geography/staff/clark_chris/britice.html which shows the unglaciated areas of Wales and Southern England during the Devensian. But what about earlier glaciations? The sarsen erratics on Fyfield Down in Wiltshire are a result of earlier glacial action - could not the same be said for bluestones in southern England?

Sorry, playing Devil's advocate here, but can't afford to leave any stone unturned ;-)