Gas sensing apparatus

To detect and locate leakage sites along utility gas pipework safely, an optical gas sensing apparatus is deployed in a cable ducting enclosure proximal to the pipework. A sensing system comprises a tunable diode laser 320, a control unit therefore 310, a receiver module 360 with at least one photodetector 362, a housing forming a sensor cell 334 having an opening to allow gas in, a first optical fibre 340 for directing light to the housing, a second optical fibre 350 for directing light to the photodetector. First and second lenses are on the sensor housing. The sensor housing is arranged to have a longitudinal axis and a cross section diameter less than the enclosure diameter such that it can be moved through the enclosure to locate gas leaks. The system may be a TDLS system to detect infrared absorption of the gas in the housing.