Hi Liz
As you have a copy, I would contact the Sheffield Archives and explain the situation to them and ask them if they could check the date of the wedding in the actual parish register that is deposited with them.
At the same time ask them if they could give you the names in the register before and after the wedding along with the dates and see if you can find these in the indexes.
The marriage registers should have been sent to the local registrar on a regular basis. It is a known phenomenon that some vicars were quite lax in sending their returns in so that some entries do not appear in the GRO indexes, and as in your case, may appear later than the actual quarter the event took place.
If you can, try to read Michael Foster's A Comedy of Errors which is a real insight
http://homepages.paradise.net.nz/mikefost/There is little that can be done now as the GRO do not recompile the indexes, there are so many microfiche copies out in the world along with freebmd and the subscription sites.
What the GRO may do is amend the master copy they have there if you can prove that the date of the marriage is 1880 not 1882 and make a note on their records.
You could also contact the local register office and let them know of your dilemma. Again there may be nothing they can do because of the passage of time.
If you can prove that the date is 1880 not 1882 then you could add a postem to the entry on freebmd. As mentioned before, the volunteers at freebmd transcribe what they see. It is no-one's job to correct the official indexes.
Dawn