From the administration of Caleb's estate it was valued at around £100 so the family could have erected a gravestone, but if one was erected of any permanence it is likely to have been removed by 1877 when the ground was laid out as a garden, so only a time period of about 33 years when few churchyard monumental inscriptions were transcribed (St Mary Newington would be an example of an exception done slightly later) apart from those of the wealthy since their monuments sold books as other people who could afford to purchase such books were interested in the families of their own class.
A transcription of St Mary Lambeth monuments was done in 1826 and published - online link given in the guide to the book which is now out of copyright but can be viewed. I don't know of any done for St John (certainly couldn't find any online but then it was not such a major church as St Mary's which was the original historic parish church) but you can check with Lambeth Archives for certain - that's the advice given in the burial guide always check local archives for possible indexes and monumental transcriptions that they might hold or know about. The links are given to each archive - Lambeth's is given here.
Most Londoners viewed the central London area churchyards as very unhealthy places which indeed they largely increasingly were from at least the late C18th onwards. They were believed to be the cause of many of the diseases and deaths in the area (bad air) and so many Londoners were pleased to see them gone and if not built on turned into gardens and open spaces. St John the Evangelist Waterloo Road was only consecrated in 1824 so would not have been a churchyard that had decades of overcrowding burials. http://www.british-history.ac.uk/report.aspx?compid=47034
A contemporary description of St Mary Whitechapel churchyard below see link - there are other such descriptions of the state of different churchyards on the London Burial Grounds website which give some indication of the situation in London churchyards in general - in the five year period either side of 1844 at St John the Evangelist Waterloo Road there were only around 6,500 burials and only just over 17,000 burials in the whole of the 30 years it was open - bearing in mind only is a comparison with other churchyards and considering the size of the actual churchyards an awful lot in such a short time span.http://www.burial.magic-nation.co.uk/bgwhitechapel.htm
St John photographed from the back giving more of an indication of the space around the church - which may have to a certain extent been encroached on overtime though contemproary images don't give the feeling it has been by much.http://en.wikipedia.org/wiki/File:St_John,_Waterloo_Road,_SE1_-_geograph.org.uk_-_1871891.jpg
and more contemporary imageshttp://www.rootschat.com/links/0flk/ http://www.rootschat.com/links/0fll/
Major city churchyards before they closed and large cemeteries were opened were very different places from their rural or market town counterparts. They are however today, those that remain as laid out gardens and green spaces, very important to London.
St John the Evangelist Waterloo Roadhttp://www.flickr.com/photos/32445100@N03/4732777298/in/set-72157623787868515