(((mtsgrad)))
As junction-guy said, perhaps a headstone was just too expensive. Even a basic funeral service isn't cheap and at a guess you'd be looking at around another £300 for even a basic headstone. Alternatively, it could have been damaged or become unsafe and been removed, although they usually just lay them down in these cases.
I like changeling's idea of closure - maybe just a simple engraved plate mounted on wood, not so extravagant as to annoy your family should they ever visit, but enough to make a statement.