I'd also like to see Isabel's instructions, but in the meantime, if you know the parish or town you want to search this is a way of doing so by batch number: http://freepages.genealogy.rootsweb.ancestry.com/~hughwallis/IGIBatchNumbers/CountyIsle_Of_Man.htm

Just click on the C (for Christening) or M (for Marriage) number for your area, and it will ask you to put in the surname. After looking at the list of names, if you click on "refine search" it takes you to a Custom Search IGI page which allows you to put in as much or little as you like, so long as you leave the batch number there.

I'll be interested to know what you think of this method, if you try it?

Sue