On FindMyPast I have found them in '61 transcribed as
William Conna, Head, Marr, 51, Collier, born Llanddaniel
Susan Conna, Wife, Marr, 38, born Holyhead
Ellen Conna, Dau, 13, born Llanfihangel Esc
Address - 1 Morfa Mawr, LLlanfihangel Esceifiog
The surname should read Evans and Ellen's age should read 1 not 10!! William's age, 51 (born c.1810) is correctly transcribed. Hence the value of providing a Census reference.
Susanah Evans and Ellin are at the same address in '71 but puzzled by Susannah describing herself as "Formerly Ag Lab wife"? As mentioned in my previous post I have found a William Evans, born 1802, Llanddaniefab - occupation Cowman.
Ordering marriage cert - have you clicked on "Place an Order"? You will need Volume and Page Number. There is no guarantee the marriage cert will deliver William's father's details (eg. deceased or William base born)