Hi,
I can't help with a look up for the 1951 marriage, however this information could be of interest to you -
John G S Coaker birth record -
John G S Coaker
Mother's Maiden Surname - Willcocks
Date of Registration - Jul-Aug-Sep 1923
Registration district - Tavistock
Inferred County - Devon
Vol 5b Page No 536
Marriage of his parents -
Francis R Coaker
Spouse Mildred T I Willcocks
Date of Registration - Apr-May-Jun 1922
Registration district - Tavistock
Inferred County - Devon
Vol 5b Page No 881
There is also the above marriage in the name of George F R Coaker.
However on their gravestone the name is Francis Richard Coaker b.1883, Mildred b. 1902 is buried with him.
http://www.findagrave.com/cgi-bin/fg.cgi?page=pv&GRid=113325822&PIpi=83713394http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=113325822&ref=acomI can only see a birth record in 1883 for a George Francis R Coaker (no Francis R Coaker). Perhaps his preferred name was Francis/Frank as per various census. In 1891 Francis is living on Sherberton Farm which ties in with the name on his grave stone.
Hopefully someone may be able to do a look up for you to confirm these details or you may need to purchase the marriage cert.
JJ