Thanks Lodger - you help has been most appreciated.

I will now try and make sense of what you have given me, by identifying the various families and marriages, then pay another visit to Register House.
I actually drove to the cemetery (from Edinburgh) earlier today and just had a walk around to see if I could find any familiar names. The old part is in pretty poor condition but I believe that I located Sarah's grave in one of the newer sections. Certainly the 1919 death date matches that of the Millen researcher. The plot looks like it belongs to the "Maclean" family with a Janet Maclean, Thomas Maclean, Margaret Mullen and Sarah McCormack Mullen all listed on the headstone. Sarah's age is given as 96 (b.1823?). However, I concur with the 1826 birth date, as she was listed at age 14 in the 1841 Census.
I'm certain that the McCormicks were not Roman Catholics as I have records for several of the marriages of Alexander/Isabella's children, two with banns in the Established Church of Scotland at Cambusnethan and Larkhall, one in the United Presbyterian Church Larkhall plus Edward's 2nd marriage in the Methodist church, Dalziel. One of the son's wives was actually employed as a domestic servant at Cambusnethan Manse.
Thanks again