Spikey, if you're in the UK you don't own the burial plot. You buy an exclusive right of burial for a fixed number of years, usually between 25 and 75 years. This can sometimes be renewed by a relative.
If I were to buy a plot for me for, say, 50 years even if there is nobody to tend my grave I will lie there undisturbed for 50 years. (Unless there is a compulsory purchase of the land for a new railway, as an example)
At the end of the 50 years the rights of burial can be sold again and my remains would be moved, or my remains may stay in situ and another body buried on top.
Good site here:
http://www.exeter.gov.uk/index.aspx?articleid=4680