Sensor/transducer for low pressure gas measurement?
I'm no expert on the plumbing/safety aspects, but I think you are probably looking for something like this sensor from Honeywell



The above came from Farnell, where there are 677 other options to look at.
Many of the expensive ones are incorporated into some metal piping which I imagine can be inserted in between or on a tap off (depending on design), and some of the cheapest are nothing more than an IC with a small tube attached, but I'm not sure how you could use these safely.

I would ask your plumber about suitability before purchasing anything, you can download a copy of the datasheet for measurements/specs.
Digikey/Mouser/RS/etc are likely to have other options also.