I have 7 relations who lived in the Rhondda but who were cremated presumably at Glyntaff, in the early 40's - 70's, all buried in a single standard grave with standard headstone at Glyntaff. The ashes of 3 people were all buried at the same time about 20 years after their death, presumably after having been kept by the son whose remains was the last to be buried of his direct family. Glyntaff crematorium had a record of the burial of cremated remains but said they do not record dates of death, although they didn't have a record of date of cremation either. So it would be worth emailing them to ask them to check if they have a record of any ashes being buried. Otherwise if they were not originally from Blaenllechau, could they have been buried at their place of birth ?