Sorry about not replying on the PM, been a bit under the weather again.
Yep, those are the sensors I am using, though I am using multiple types (MQ2 and MQ6 together and have MQ135 on order). Just got another batch of both in the mail today, ordered them a while back from a vendor in China.
Yep, that's the basic idea- the technology is pretty simple.. it's getting this thing made and implemented and certified that is the real issue. A commercially produced and certified valve that would be installed on the mains- which you and I are very much not allowed to do. In many places it's illegal to mess with the gas, in all places it's foolish!
Development of the hardware really isn't a big deal, nor is the code. It's putting the thing together and getting it in front of the right people to make it happen that is the big battle 