Don't rush going to the regulator, once you have fired that shot you are out of ammunition.
Does Bord Gais operate under a licence, what are the terms of that licence. Can you base your complaint in these terms.
I would be very interested to know as I have similar issues with telecom providers.
In my experience making complaints to banks, there is great leverage in the internal complaints procedure, and the threat of going to the regulator. The will make considerable efforts to stop a complaint going to the regulator, however once the complaint does go to the regulator they have nothing to loose by defending themselves vigorously, and not much to gain by fixing your issue.