Hi Dave
You mention that it appears to be currently the case that in Ceredigion that rights last 100 years. I am not aware of any local authority in England that grants the max that the law allows for. It is usually 99 years and many in England have moved to only granting in some instances 30 years...even on triple depth graves.
My father had been granted 99 years for the double depth grave that he bought when my mother passed away. My father was later buried in the same grave and the rights passed to me. I later lost my son and I bought the exclusive rights for a single depth grave. I took it for granted that the rights was the same and never read the document when it arrived. I only learnt last year that 30 years had been granted. I never imagined that I would need to address in my own lifetime renewing the grant on my son's grave. I think that this is appalling which made me begin examining what other local authorities grant under the law.
I learnt that remaining space in consecrated graves are being reused in the City of London cemetery and being resold as Heritage graves. The City of London has applied for a faculty to move to the lift and deepen method. For any readers that do not know what this practice is, it is where the body is removed, the grave dug deeper and the body replaced...freeing up space to sell more graves. I am not aware that this method has been adopted yet, but it will for certain and one can be sure that when it does happen that this practice will be adopted by local authorities across England and Wales.
In respect of of your understanding of extending burial rights for a longer period if there is proof of relationship, this is not my understanding. I do not believe that relationship has anything to do with rights granted, but you would need to read the Local Authorities Cemeteries Order yourself or maybe another member of the forum will provide the answer. If this fails to be the case I would have a chat with an officer at your local authority.
teresaevans