why get a certificate ? - borrow the LDS film of the parish reg which is a film of the official transcript used to produce your certificate ?
At that period I would expect little if any additional information to be in the PR.
Likewise for the marriage certificate - all the info is in the official transcript of the parish reg on film - probably just the witnesses.
However if you do wish to help reduce my Manx tax bill look at the web page http://www.gov.im/registries/general/civilregistry/welcome.xml - you already have it seems all the necessary info to order.