Hi Sam,
I'm originally from Cwmafan, the part of the graveyard was built over with a "new" housing development, they created a memorial garden with some of the headstones, I can remember as a child it was not cared for very well, but I do believe that only a selection of headstones were erected in the memorial garden.
The old cemetery was a part of. St.Michaels it was separated from the church by a railway line now long gone. If the grave you are looking for was in this part there must surely be a record of removal of headstones. I too I'm stuck looking for some of my Ancestors graves at St Michaels, wishing you the best of luck with your search.