I know this is an old post now but always useful to supply information for future reference in my opinion
The sextons house is long gone now and i have tried locating his plot map but no one seems to know where it went. Glyntaf office i find a bit hit or miss. The best resource is Treorchy library in my opinion. Most people, including the library, are using the CD made by the GFHS for searches, this does contain quite alot of errors though!!
I have been doing my own detailed map over the last 6 months and photographing plot by plot, creating a resource of my own.
What needs to be considered is that J23 could be in two places one in consecrated and one in unconsecrated ground, unfortunately the CD does not differentiate! they are however on the original records. There are a John Williams and Edmund Morgan buried in the unconsecrated ground so unless they are related then they must be in the consecrated ground, unfortunately i have not yet mapped this section or photgraphed. The E54 plot can relate to three different graves (unconsecrated, consecrated and PLOT E) i have indexed this one tho and its in PLOT E which is near the back left, confusingly someone has placed a later grave marker at this plot with two people, i have to therefore assume these are related to you. David John Jones and Catherine Ann Jones (i have image if you want)
Hope this helps, if my reply is still relevant.
David