borrowed from robert tyrrells post..
Below is an extract from the GSIUR:
Gas work should only be undertaken:
(a) by a person who has successfully completed an industry-recognised training course followed by assessment of competence. Training that leads to assessment of competence in safe gas work should be recognised by the industry’s standards setting body; or
(b) in the case of a currently or previously registered person, where they have proved competence through a certification scheme; or
(c) for those working at premises that fall outside the scope of the Regulations (see regulation 2(4) and associated guidance), by a person who has successfully completed an appropriate full training course followed by assessment of competence.
The mention of competency refers to the ACS exam, and registration with Gas Safe Register