The marriage certificate is the same no matter where you order it from.
In theory that is true, but not always in practice. I'm sure I'm not the only person who has certificates that differ slightly in the GRO copies from the local office copies. The registers at the local offices were copied quarterly and sent to the GRO. Therefore, GRO certificates were rewritten an extra time, and as whoever copied them out was only human, there are bound to be errors. The local register copies are a step closer to the original than the GRO copies.
Vivien, did they marry in a church? If so, it would be worth tracking down the parish register. It should in theory contain the exact same information, but for marriages in churches, the PR is the original source of any info on a GRO marriage certificate.
So there is a chance a mistake was made during the copying process from the parish register to the local register to the GRO register.
Alexander