Guernsey Marriage Lookup request

[kady]

I am seeking the marriage of my 2xGreat Grandparents - Thomas Bolton CUMMINGS and Mary Ann OSBORN(E).  They raised their family in Jersey (St Helier) but their marriage couldn't be found there.  Her family lived in St Peter Port, Guernsey, so it is possible that they married in her parish.  We are assuming that the marriage would have taken place around 1866 as their first son was born in 1867. 
If anyone is able to locate their marriage, I would also like to order the certificate so any info on how to do that will also be appreciated. 
Thanks so much!
Kady