Dear John,
I found Mary Ann Ketteringham on the 1851 census returns aged 5- she was the sister of my great-grandfather, Robert
Bristol Ketteringham. I seem to remember seeing the name
Mahala when I did the search, and it is quite possible, as you say, that there was another Mary Ann who was related and who could have been the same sort of age. I have also found a link with the name
Howe. On the Births, Marriages and Deaths Index (FreeMBD) on Rootsweb, there is a marriage between Isaac
Howe and Mary Ann Ketteringham in the March quarter of 1869 in the
Durham district (ref: 10a page 391).
Best wishes, Ruth