If you are thinking of a personal injury claim, there is no claim for nearly being killed or could have been killed. There is only a claim if you suffer injury.
You would need a toxicology report confirming the effects of the gas on you and its long-term effects.
Research suggests that gas itself is not toxic as such and what people die of is carbon monoxide poisoning (which is something different altogether, a byproduct of combustion) or asphyxiation from breathing the gas.
You could try approaching no win no fee solicitors but I suspect that you would struggle to find want to take this on for you.
Regarding the cooker, this was dangerous and although you make the point that it was never used without supervision, it was in itself an inherent danger and hence, the gas fitter was correct with his actions.
I’m sorry, I know it’s not a favourable answer but that is the situation.
Can I clarify anything for you?
Please don’t forget to rate the service positive. It’s an important part of the process by which experts get paid.