The 1851 census gives Mercy's maiden name and the reason for the different surnames given for Mary Ann Farey/Waring on the 1881 and 1891 censuses
1851 census HO107 1721 folio 526
Church End Buckland Buckinghamshire
Thomas Waring 48 Buckland, Buckinghamshire, Head Married Ag lab
Sarah Waring 42 Soulbury, Buckinghamshire, Wife Married
Rachel Waring 23 Buckland, Buckinghamshire, Daughter Straw plaiter
Martha Waring 21 Buckland, Buckinghamshire, Daughter Straw plaiter
Mercy Waring 19 Buckland, Buckinghamshire, Daughter Straw plaiter
Ann Waring 15 Buckland, Buckinghamshire, Daughter Straw plaiter
Thomas Waring 13 Buckland, Buckinghamshire, Son Ag lab
Elizabeth Waring 11 Buckland, Buckinghamshire, Daughter Straw plaiter
Jonah Waring 9 Buckland, Buckinghamshire, Son Straw plaiter
Philip Waring 7 Buckland, Buckinghamshire, Son Straw plaiter
David Waring 3 Buckland, Buckinghamshire, Son
Eliza Waring 9mths Buckland, Buckinghamshire, Daughter
Sarah Waring 6mths Buckland, Buckinghamshire, Daughter Child
Marriages Dec 1852
Dunton Joseph Aylesbury 3a 633
Keen Elizabeth Aylesbury 3a 633
Waring Joseph Aylesbury 3a 633
Waring Mercy Aylesbury 3a 633
Regards
Valda