My unit works ok and can be adjusted low for simmering. There are larger burners that would be an issue for smaller pots at low simmer. As for the wok burner, its not going to compete with a restaurant burners heat energy. It does work as expected for me but yes, you would not be too ambitious on its ability. The burners at low setting should be just on and very low flame near the ring. . All the gas fitter can do is make sure it is operating at the right design supply pressure and that the right gas injectors have been fitted for the type. Eg natural gas or LPG. Many units come with additional gas injectors if needed to convert over but if you have ordered it as the type needed, it should have been supplied correct.

The builder reckons the gas fitter did what he had to do. We had gas in our last house and the wok burner was much better. My wife has cooked with gas almost all her life so she is not just a beginner. There is something wrong. It seems to be very hard to get someone to even look at it.

If this unit was working ok from the start and now has gone south, my only thoughts would be the operational pressure of the unit. All this unit has in it is an electrically driven ignitors, basic flame failure thermocouple protection and injectors. If all are odd, it seems to me to be leading to that one thing. Seems to me that you need independent advice from another gas fitter.