Apparatus for converting a flexible liner 2 including a layer 3 of composite material comprising thermoplastics material and reinforcing fibres, into a structural member within a duct 1, has a front portion 6 for insertion in the liner 2, a central portion 7 for heating one side of the layer 3 and a rear portion 20 which forces the heated layer 3 against the duct 1 for consolidation and cooling under pressure. The central portion 7 has heating means 8 producing hot gas which is forced under pressure through the layer 3 of composite material to heat the layer 3, and provides an air gap 14 on the opposite side of the layer 3 while heating takes place. This ensures uniform heating of the layer 3, to melt the thermoplastics material which then mixes intimately with the reinforcing fibres, and is consolidated and cooled to form the structural member.