A test tool assembly (10) for location in a section of pipe (12) comprises an isolation plug module (14) for isolating an upstream pipe section (16) from a downstream pipe section (18) and a test plug module (20) which permits a pressure test to be carried out on the upstream pipe section (16). The assembly (10) is configured so that a force acting on a seal arrangement of the test plug module (20) as a result of a test pressure applied in the pipe (12) is lower than a force required to maintain the seal between the seal arrangement and the pipe (12).