The Road Vehicles (Construction and Use) Regulations 1986

15.  Every part of a gas propulsion system or a gas-fired appliance system, excluding the appliance itself, shall be— E+W+S

(a)so far as is practicable so located or protected as not to be exposed to accidental damage,

(b)soundly and properly constructed of materials which are compatible with one another and with the gas used or to be used and which are capable of withstanding the loads and stresses likely to be met in operation, and

(c)so designed and constructed that leakage of gas is unlikely to occur.