Grave boards at Ayot St Lawrence from a postcard published circa 1905.
I frequently get requests from people who have discovered that their ancestor was buried in a particular parish and are planning to visit - so a brief summary of what they might find is appropriate.
Before about 1800 nearly all burials would have been marked with wooden grave boards, such as the above examples, and some were still being used a hundred years later. Typically after 50 year the wood would be rotten and for many centuries the grave would have been renewed. The very few graveboards that remain will be completely unreadable unless they have been renewed.
While a few late 17th century grave were given headstones they typical materials used in Hertfordshire mean that often it is not even possible to see if there was an inscription. During the 19th century the use of gravestones became common - and in most of the towns the churchyards fill up and town cemeteries were opened - and burials ceased in the church graveyard.
Maintaining a graveyard full of stones of the long deceased can be an expensive business and many church graveyards have been tidies up - particularly in towns. This may (if you are lucky) means laying the stone flat - inscription up - so it can be mowed over. In other cases stones are leaned against the churchyard wall, used to make paths (and in one case I have seen the walls round a compost heap) or taken to the tip. - Some 50 years ago I was horrified to find that the beautiful pink granite slab on one of my great great grandfather's tombs had been scrapped in this way.
So if you are planning a visit be sure to check the burial register (at HALS or online at FindMyPast). Find out if the Herts Family History Society have indexed what is left. It can also help to look for online photographs of the church (Google satellite or street views, Geograph, etc) to see if any tombstones remain.