I have done more investigation. My terrain has come from world machine and imported via Import Tiled Landscape, but it seems that the collision on the imported terrain is the problem, not the Origin Rebasing as I have exhaustively tested with and without this feature turned on (via the World Settings when the level is open).
As such I shall go scour the UE4 forums to see if there is a solution for collision on imported terrains, since having my character up to his neck in the terrain is far from satisfactory.