Gas pipe fitted in trench in back yard going to garage - is it legal, do I need certification etc?

first of all the "guy" who did the work needs to be qualified for relocating/installing gas meters. This should be easy to check as it will be on his card, if he has one. otherwise go into gas register website and do a check there. There are strict rules with running of gas pipes and installing new gas meters, lets hope he has not thrown some copper on the ground and called it "job done"!! To answer your question, yes you do need the work to be certified. hope this helps ben