I've no links to share right now, but the dismissing the "Celtic descent vs German descent" idea doesn't make much sense to me. There are traits that can be followed genetically which "belong" to each group ("belong" being used loosely, since they are all fundamentally mutations). There was a study on TV once where Norse genes were tracked down throughout Britain, and another study where a certain gene which belonged to Genghis Khan was tracked down throughout Britain (the number of people with the gene was surprisingly voluminous). It's all very interesting stuff.