A leak clamp assembly for repairing a leak in a spigot and socket joint in a gas pipe includes a split sealing ring of resilient material which is placed around the spigot and against the outer face of the socket and a split rigid backing-up ring which is also placed around the spigot on the side of the sealing ring remote from the socket and a series of clamps which act between the backing-up ring and the back face of the socket. The backing-up ring comprises at least two arcuate sections containing a bolt hole at each end, the sections being connected end to end to form the ring and overlapping at their junctions so that the corresponding holes are at least partly in register with each other. The clamps comprise bolts for passing through the pairs of overlapping holes and designed to engage behind the socket preferably with an anchor ring which is assembled around the pipe behind the socket. The bolts comprise a conventional shank for cooperation with a nut and a head which tapers to the shank, the arrangement being such that the backing-up ring section can be arranged with the bolt holes overlapping to a minimum degree around the shanks of the bolts to provide the backing-up ring with a maximum size, but on tightening the nuts on the bolts, the tapered heads are drawn into the overlapping holes thus increasing the extent of the overlap and drawing the backing-up ring sections further together until the backing-up ring tightly engages the sealing ring and clamps it around the spigot and against the face of the socket of the spigot and socket joint.