The Anglo-Saxon Chronicle records the first substantial Danish raid in Kent in 835, when "heathen men ravaged Sheppey". Later, in 855, the island of Sheppey became a more permanent base from which Viking raids into Wessex and other kingdoms could be launched.
Fenton, Albert. "Danes raid Kent". The Literary Encyclopedia. First published 06 April 2016
[http://www.litencyc.com/php/stopics.php?rec=true&UID=1932, accessed 23 August 2016.]