An insert for use in connecting two lengths of pipe, each having a plastic liner. The insert consists of an inner tubular member of plastics material for connection at either end to the plastic liners of two pipe lengths, an insulating sleeve surrounding a central portion of the inner tubular member and a ring of heat-resistant material overlying a part of the insulating sleeve to be positioned, when in use, beneath a location at which the pipe length is butt welded, wherein the insulating sleeve includes an insulating material which is at least partially enclosed with a sheath, such that the sheath can be mechanically connected to the interior of one pipe length.