Headstones are usually fixed using the monolith technique, where the headstone is buried in the ground, as opposed to sitting on a stone plinth. It depends, of course, on the regulations of the church or cemetery. The monolith technique involves making an 18” hole in the ground and placing the headstone on two blocks acting as a foundation; the hole is then backfilled and compacted. Concrete is never used, as the ground always moves, and if the headstone tips slightly and it has a heavy concrete base, it would be virtually impossible to upright again. For further information see my workshop news entry on fixing headstones.