It should be commissioned , signed off and fitted by a G3 qualified engineer and going through building control will depend where in UK you stay and whether the work has a building warrant.
how do I check and do they come round and check the system. I don't want to open a can of worms here and end up with the building control condemning it.
the responsible person for ensuring notification is the house owner, but most G3 plumbers are gas safe and can notify via gas safe or oftec , however our local bc dont want to know if its a replacement cyl, and dont seem to bothered if its brand new and replacing an old ov system.