Frances, my John Cowin, son of William married Margaret Skillicorn in 1844 . They had 3 children and came to Australia some time between 1846 and 1855 so he could not be the one who married Eleanor Cannell.
From a lookup by Brian Lawson, for me quite a while ago, there was John Karran, tailor, wife Margaret and 2 children at Ballagawne in 1851. This would be my John Cowin. His parents Thomas Cowin and Margaret Karran were married Jan 1806 according to IGI.
If Margaret Karran was the child of Hugh Karran and Eleanor Quayle b 1779, the dates would fit.
There are 2 other Margaret Karrans born 1785/6 , from Marown and Santan, which would also fit time of marriage . It is certainly a puzzle. Thank you for your help I will keep going slowly and try to verify all I find. There are so many sites out there that just copy each others info without checking, which makes things hard for all.