A sealing plug for sealing a hole in a wall, which sealing plug is insertable into the hole by access from only one side thereof, which sealing plug comprises: a radially expandable body which is radially expandable by access thereto from only one end thereof, and a sealing cap which covers the other end of the expandable body and extends therefrom along the exterior of the length of the body, the sealing cap being of a material which is fluid resistant; whereby, when the sealing plug has been inserted in the hole and has been radially expanded so that at least part of the sealing cap adjacent the wall of the hole expands and engages therewith, the sealing cap is secured in the hole with the sealing cap sealing the plug body against access thereto from outside the cap.