Not a lot of help but I can give you some (very limited) information on Sarah Whitmarsh. I am carrying out a one-name study of Whitmarsh. I have collected all BMDs from 1837inwards and also censuses from 1841 onwards plus some parish records for earlier years. I have very little information about Sarah or her family - they seem to be very elusive. She appears in the 1841 census with her husband Thomas, a navy pensioner, living in Cann. He died in 1844. To arrive at the granddaughter you found in the 1851 census I have had to assume Sarah had a daughter who married an Allen but I have no evidence to support this! This might save you an abortive search.