I do remember it being difficult to find out exactly how one requests a certificate from IOM - and being from Australia it was made more difficult by the fact they didn't have credit card facilities and it would have cost me as much to get a bank cheque in pounds as the certificate itself! In the end I went through a certificate service person (I'm sorry, I can't remember who I used).
I think you might just have to contact them to find out how to do it - they definately do not have an online service, more's the pity. I'm pretty sure they sent me a paper form to fill in.
Alternatively, if you have a local LDS centre, you could see if the marriage entry appears in the parish register - I think this is the most likely film:
"Register of Church of England baptisms, burials and marriages in: Kirkonchan ... Marriages 1886-1911 ..." Film no. 454980. According to Brian's site these registers have the same information you would expect to find on a certificate (but I have never seen these films so I couldn't say for sure).
Cheers,
Ruth
