Gas sensors and detectors; gas detecting, monitoring and analysing apparatus and instruments; controllers for gas sensor and detector systems; apparatus and instruments for controlling gas sensor and detector systems; apparatus and instruments, all for detecting combustible gases; transmitters and receivers of infra red radiation; gas testing instruments; optical infra red apparatus and instruments; optical, infra red apparatus and instruments; electrical and electronic sensors for gas detecting apparatus and instruments; safety apparatus for gas detecting, monitoring and analysing apparatus and instruments; parts and fittings for all the aforesaid goods