As you say Geoff there are two different methods of recording the plot, i have finally found the MI listing for some of my relatives as compiled by the FHS and available in the library.
The listing is for the Old Cemetery and the plot reference is N37 (Last entry on the lsit below, surname of Hay). Clearly there isn't a section "N" on the board as you enter the cemetery but the listing is in a loose cover which has an outline of the section and all the 31,32,33...... numbers noted to indicate where they are.
The loose covers are useful but for plots on the right hand side of the cemetert it is more difficult to place them accurately as the sections "c" and "d" as shown on the board are almost mirror images of each other.
The cemetery register gives the plot number as A740 (i found the details when familyhistoryonline.net was still functioning).
The location of the plot is marked below,
Looking back over the thread i did post a picture of the stone (13th of September), the MI listing and the headstone only show two burials, the cemetery register shows a third burial in the plot, it's only by combing the MI listing, cemetery register and a bit of legwork that i pieced it all together, the MI list alone might not be enough to locate a grave and it's also worth remembering the MI list was comiled some time ago and lists some stones which no longer stand.
