Hi Sy
Very few plots in municipal cemeteries are purchased 'in perpetuity', most are leased for 50, 75 or 100 years.
The cemetery offices do know who the registered keepers are/were in case the deeds to the plot have gone missing. I do know that if you are not next of kin, but are related to the family interred, you can get legal documentation to have the deeds transferred into your name if the lease hasn't yet expired.
I understand that each municipal cemetery will have its own rules and regulations as to if a new memorial can be erected or plot re-opened.
Many of the older cemeteries around England & Wales where plots haven't been re-opened or the leases expired years ago are now designated as areas of conservation or nature reserves and work cannot be carried out in them.
What is also starting to happen in and around London where there is a massive shortage of space, is that some of the older plots, where the leases have expired but where there is still room for more interments are being re-used by other families who want to have family members buried rather than cremated. There have been discussions on here about it. What I would say in the cemeteries' defence is that that do try to make contact with the descendants but the passage of time since the last interment, expiry of the lease and today is huge and they do not have the financial resources to do the detailed genealogical work required to track down and make contact with living relatives.
Dawn