I know roughly where the headstone would be, it is next to a that for Redpath & Coleman which is Row R no 37 and your grave is Row R no 38, this is for the James and Margaret burial. The New extension area was to the right hand side of the Church as you are facing the door with the Garage behind you. It is the area that runs all down the right hand side of the Church Yard and the grave you want is partly under a tree if the tree is still there, you would have to check them out, haven't visited since 2019. You would be best view the cemetery register that gives details of graves and making a note of other graves in the Row, I know there are Algate & Oates headstones in it as well, Sadly many headstones have gone from this Churchyard over the years.