The English in general (not just the south-east) have about 45 to 50% German/Scandinavian type DNA but the Welsh and Scots have similar amounts as well, so the problem is trying to work out how much of the Germanic stuff was already in the Celtic/Roman Britons before the AngloSaxons and Vikings came. But the two most recent studies both estimated a contribution of around 25% from post-Roman invaders/immigrants. That seems a reasonable guess to me, taking into consideration the change in language and culture during the Dark Age.
I've posted this map a few times but you might have missed it:
http://i657.photobucket.com/albums/u...pswv58tnlo.jpg