Sorry got a bit confused initially with the Couch name.
A William Murrin married a Mary Couch in 1802 and they had 8 children, including an Edward and a William.
Edward married an Ann(e) Garde in 1832 and William married an Ann Warrell(in 1842.
A Mary Couch Murrin (born 1849) seems to be living with Edward and Ann(e) in the Census years of 1851 and 1861 as their daughter.
However looking at the Baptism Register for 29 April 1849 the parents name for Mary Couch Murrin (bap. when she was 3 weeks old) is clearly given as William and Ann.
But the GRO index has the mothers maiden name for all of the children of William and Ann as being Warren except in the case of Mary Counch Murrin where GRO says the maiden name of her mother was Gard.
Haven't got a birth cert. but this suggests to me that she was actually a daughter of Edward and Ann, and that the named parents in the Register were actually an uncle and his wife.
Apologies for having erroneously suggested those named were grandparents but in any case the Register gives no indication of the true relationship of those named as parents.

David