If it can be proved (and I’m not doubting you for a moment) that the replacement cooker caused the damage before you moved in then it’s pretty clear to me that the house builder gave you possession of a house in ‘immediate danger’. Potentially an ID situation that is reportable to the HSE. That is to say a gas leak is an explosion waiting to happen. That no explosion happened is beside the matter.
Having fitted many kitchens as part of my work I can tell you that the hob gas pipework should be out of the way of the oven so that it cannot be damaged. If the replacement oven was somehow larger than the original then the fitter should reasonably have measured it and had the pipework adjusted rather than just ramming it in!
In my recent training (and I hope I’ve got this right), I’ve learnt that natural gas will rise and carbon monoxide will fall. Carbon monoxide is a killer but you haven’t described that as your problem. Natural gas will rise and possibly escape through open windows and doors…or it might not and could become sufficiently concentrated to be an explosion risk.
Seems to me that the builder should accept that they have created a dangerous situation, cover the costs of testing and repair and make a contribution towards your gas costs. Getting an admission of liability might not be easy. One thing is for sure, they won’t want the HSE involved so I would mention this in any correspondence.
I think you’ll have trouble calculating the quantity of gas lost. The accurate way of doing it is also a very dangerous way which requires you to turn on the gas to the leaking pipe and measure the gas used at the meter over a period of time. I can’t overstate how dangerous this is and must not be attempted.
Your Gas Safe man could write a statement and provide pics of the damage to show how a careless cooker I installation has created a danger.
Hope this helps.