So sorry, seem to have forgotten about this thread and have only just remembered about my original post after googling my name.
Thank you both for your suggestions.
William Cowell did indeed move from the Island: the 1851 and 1861 censuses see him in Whitehaven, Cumbria with his family but he seemed to have connections to Liverpool as his wife and daughters were there on the 1841 census. Liverpool is also the place where one of his sons died. I suppose that LIverpool is a logical conclusion seeing that he was a merchant mariner. Unfortunately all trace is then lost as his wife was widowed by the 1871 census. Any suggestions would be much welcome. - Petra