Ring 999 and you get asked which service you require, perhaps the question should be more open, i.e. what is the emergency.
On that basis, I smell gas, can be directed to National Grid, Northern Gas Networks, or the appropriate of the other local companies.
In response to cudzos post, yes gas leaks can be a public safety issue, but in my experience, a small local failure is dealt with entirely under the regulatory response times, and risk assessments, COBRA etc is only going to get invoked for major failures affecting thousands of people, and the companies themselves have mutual aid agreement. Risk factors for response to leaks include location, proximity to vulnerable persons, size and pressure of leak etc I know this because I designed the protocol.