Nigel - I presume you have read Proc IoMNH&ASoc vol XII #3 pp469/486 "A bioarchaeological study of the human remains from the early medieval cemetery of Cronk Keeillane" by K A Hemer which showed (via analysis of isotopes in bone etc) that over half had been born (or at least spent their childhood) off Island and derived from Ireland and various places in mainland Britain - possibly having moved to IoM to avoid the plagues which swept across the British Isles in 6th + 7th centuries AD - 'migration is not an exception, but a constant'