Don't forget that with church marriages there is an alternative source: the parish register. The PR entry should contain the same information as the certificate, and as a bonus it will be in the original handwriting, unlike the GRO copy.
If you know the date and place of the marriage, the record office might supply a photocopy for a small charge if you can't get there yourself. It's worth asking. Obviously it will cost more if a lengthy search is involved, but rural weddings are usually quickly found. More difficult to track down in a city with lots of churches.
