Having now looked at this more carefully I have to point out that you are making assumptions about relationships. They are probably right but... the 1841 does not list relationships therefore it is absolute guesswork to say Birkett is a grandson. There are other young women in the household. Ann age 27, Mary 25, Jane 18, Margarett 16, Sarah 10 and Jno?? 20 (male)
I am sure you are correct but the 1841 does not prove it. As there is a Margarett that makes your other census entry a bit more hopful.
Have you looked on family search to see if any of these children show up?
Wills may help here as if they were farmers they may have owned land.
Is it possible for you to get to the record office? A search of the local parish register may show up burials and marriages that are not on the IGI. You can order the film of the nearest parish church from the LDS and search it at their centres if not.
Andrea