Mary Whitehead is kneeling in front of a part buried gravestone thick with yellow lichen when she calls me over to take a look. We are in the churchyard of the 12th century St Michael and All Angels Church in Fringford, just north of Bicester, and the decorative carving on the stone’s crown as well as its positioning on the main pathway tells her that this might be one of the oldest here. Carefully she takes her trowel and pulls back the inch or so of turf that has grown up around it.