When you get married in church there are three books. Two marriage registers, one for the Church Records, and one subsequently sent, when full, to the Register Office, these are signed by the couple and the witnesses, the third book is the marriage certificate, filled in by the vicar as a certified copy, and given to the couple, they should all have the same details. In addition to these books are the forms filled in for the quarterly certified returns. So there are four documents where errors could occur.
The Registration Supplies Unit at the GRO provide the necessary registers etc. These include marriage registers, in duplicate, forms for the quarterly certified copies, and books of standard marriage certificates for issuing to the public.
Stan