Hengest and Horsa may well have been characters in a story rather than real people - their names apparently mean 'horse' and 'mare', and there is nothing that archaeologists can actually pin down as 'Jutish' rather than early Anglo-Saxon / Migration period. The tale of H & H being invited to settle then taking over an area may be a microcosm of events that happened repeatedly over the rest of Britain as Roman rule broke down.