 In 851 , a Viking army landed at Thanet , then still an island , and over @-@ wintered there . A second Viking force of 350 ships is reported by the Anglo @-@ Saxon Chronicle to have stormed Canterbury and London , and to have " put to flight Beorhtwulf , king of Mercia , with his army " . The Vikings were defeated by Æthelwulf and his sons , Æthelstan and Æthelbald , but the economic impact appears to have been significant , as Mercian coinage in London was very limited after 851 . 
