I don't know what type of gas meter you have in uk so this might not work for you but I used a aqara door/window sensor without the plastic case stuck under the meter digits. There's a magnet embedded in the last digit so it will trigger it once per revolution. It looks like this: https://community.home-assistant.io/t/gas-meter-from-xiaomi-... .I saw that others 3d printed an enclosure and moved the reed sensor closer, but, i just wrapped it in electrical tape and it's been sitting there for 6 months without an issue.