I think you were 'sold' a solution to a problem that didn't exist. As the attached link Q&A makes clear, these things have to be specifically designed for the tile system used - yours may have been since you said it was part of a complete reroof and not a simple retrofit.
http://uk.answers.yahoo.com/question/index?qid =20061025085313AAcWh5Z
I guess the answer to your question about using mortar is that it depends on the size of the gap that now exists - mortar has traditionally been the way such verges have been sealed.
Without seeing, it is a bit hard to know.