Hi Jackie, It's possible that there was never a headstone. For many people the cost of the burial was hard enough, and paying for a headstone was simply beyond the reach of many families.
I've had a few trips to different church-yards over the years, armed with a list of people I know from parish records were buried there. I'd guess my success rate overall is somewhere between a third and a half. Once I got really lucky and found a whole corner of a churchyard full of my ancestors, but on other occasions I've found nothing.
I'm visiting St John's at Cinderford in a few weeks time so if you give me the names I'll add them to my list and see if a fresh pair of eyes helps at all. Thanks for the warning about it being rather overgrown there!