The plug is formed from a short piece (1) of elastic pipe (2) with one end tied with a cord and the free and folded flat onto the top of the plug. The plug is pressed inside a mould (6) for a permanent shape. The elastic pipe has an inner reinforcing layer (3) and the ultimate shape is achieved by the moulding process either as a vulcanising process or by adding a hardening agent to the mould. The mould also produces profiled grooves and ridges (7,8) in the skirt of the plug to be gripped by the pipe fittings.
