Thank you Sarah for your all that new information. I don't know what details were included on certificates at that time, but it would be very interesting and possibly very helpful if someone could have a look at the marriage certificate or church register entry for William and Frances to see if their fathers are named, similarly with the birth certificate or baptism register for Anna. That's the only way I can think of to be sure we've got the right connection - hope so!
It would be France Cavendish's grandmother, Margaret Killey, that was born in the IOM as she herself was born in Ohio.
Let me know if I can help with any of the IOM ancestors though perhaps we ought to try and establish we've got the right William first!
Incidentally I just found your reply by cruising the board today - the usual reply notification didn't come through for some reason.