Hello,
There is a marriage for Edward Sadler and Elizabeth Gwilliam on the 3rd Mar 1785 in the parish of Trellech.There is also a possible baptism for Elizabeth Gwilliam, daughter of William and Mary Gwilliam on the 26 Feb 1764.
Could not find anything on William Sadler but i would recommend you google Monmouthshire Parish Record Transcripts, which is a very good source of information although not every parish is complete at the moment.
Best Wishes,
talllpaul