Robin Fleming, Britain after Rome: The Fall and Rise, 490 to 1070 (Penguin, 2011), 458 pages, $24.95 Tim Clarkson, The Picts: A History (John Donald, 2010), 192 pages, £14.99 David Mattingly, An Imperial Possession: Britain in the Roman Empire 54 BC–AD 409 (Penguin, 2007), 640 pages, $29.95 The Anglo-Saxon story takes another battering in this latest and second volume of the Penguin History of Britain. For centuries, the English and their diaspora were told they were descended from bold, Germanic tribes of Angles, Saxons and Jutes who invaded and took over a shattered island after Rome withdrew. In recent years,…