Sensor apparatus 18 for use in a plastic service pipe 16 inside a steel service pipe 10 connected to a main consists of a flexible element 42 which is fed through the steel pipe 10 and through the plastic pipe 16. The element 42 has at its remote end two light-emitting diodes 80 and two photoconductive cells 82 The flexible element 42 and the pipe 16 are fed together into the steel pipe 10. The sensor apparatus 18 indicates when the cells 82 have entered the main 14 because the intensity of the light reflected to the cells 82 falls to a low value and gives an audible or visual signal. This also indicates that the end of the plastic pipe 16 which carries a seal 38 is positioned correctly and that sealant 30 can be injected into the space between the steel pipe 10 and the plastic pipe 16 and is detected by a second light sensor arrangement 154/156. After the sealant 30 has been injected the sensor apparatus 18 can be recovered by withdrawal through the nose-piece 32 and through the pipe 10.