A plug for plugging a line, the plug comprising: a seal; a first end assembly located at a first end of the seal and a second end assembly located at an opposite second end of the seal, the first end assembly comprising at least one gripping member (16, 17) that is configured to grip an inner surface of the line; and an actuator configured to draw the first and second end assemblies together when there is a relative motion between the actuator and the first end assembly, wherein the actuator is configured to actuate the gripping member, and the plug is arranged such that: an initial part of a motion of the actuator actuates the gripping member to grip the inner surface of the line, thus inhibiting motion of the first end assembly, and a subsequent part of the motion of the actuator, relative to the thus motion-inhibited first end assembly, draws the second end assembly toward the first end assembly so as to compress the seal such that the seal may seal against the inner surface of the line thus plugging the line, wherein the actuator comprises a gripping member-contacting portion that is located adjacent the gripping member (16, 17), the plug being configured such that motion of the actuator causes rotation of the gripping member-contacting portion, which in turn causes the gripping member to be actuated, wherein the actuator comprises a threaded elongated member, and characterised in that the gripping member-contacting portion comprises threads that are engaged with the threaded elongated member of the actuator. a seal; a first end assembly located at a first end of the seal and a second end assembly located at an opposite second end of the seal, the first end assembly comprising at least one gripping member (16, 17) that is configured to grip an inner surface of the line; and an actuator configured to draw the first and second end assemblies together when there is a relative motion between the actuator and the first end assembly, wherein the actuator is configured to actuate the gripping member, and the plug is arranged such that: an initial part of a motion of the actuator actuates the gripping member to grip the inner surface of the line, thus inhibiting motion of the first end assembly, and a subsequent part of the motion of the actuator, relative to the thus motion-inhibited first end assembly, draws the second end assembly toward the first end assembly so as to compress the seal such that the seal may seal against the inner surface of the line thus plugging the line, wherein the actuator comprises a gripping member-contacting portion that is located adjacent the gripping member (16, 17), the plug being configured such that motion of the actuator causes rotation of the gripping member-contacting portion, which in turn causes the gripping member to be actuated, wherein the actuator comprises a threaded elongated member, and characterised in that the gripping member-contacting portion comprises threads that are engaged with the threaded elongated member of the actuator. an initial part of a motion of the actuator actuates the gripping member to grip the inner surface of the line, thus inhibiting motion of the first end assembly, and a subsequent part of the motion of the actuator, relative to the thus motion-inhibited first end assembly, draws the second end assembly toward the first end assembly so as to compress the seal such that the seal may seal against the inner surface of the line thus plugging the line, wherein the actuator comprises a gripping member-contacting portion that is located adjacent the gripping member (16, 17), the plug being configured such that motion of the actuator causes rotation of the gripping member-contacting portion, which in turn causes the gripping member to be actuated, wherein the actuator comprises a threaded elongated member, and characterised in that the gripping member-contacting portion comprises threads that are engaged with the threaded elongated member of the actuator.