A plug for sealing a conduit comprises a body having a first section and a second section, and at least one seal element for creating a seal between the plug and the conduit. The at least one seal element is adapted to be energized by movement in a setting direction of the first body section relative to the second body section. The plug further comprises seal locking means comprising a first portion and a second portion wherein as the at least one seal is energized, the seal locking means first portion is rotatable unidirectionally relative to the seal locking means second portion to take up the movement of the first body section relative to the second body section in the setting direction and prevent movement of the first body section relative to the second body section in a releasing direction, opposite the setting direction. In one embodiment the seal locking means first portion is a locking nut.