Find my gas supplier
The Meter Point Administration Service can tell you who your gas supplier is.

You can:

use their online Find My Supplier search tool http://www.findmysupplier.energy
call them on 0870 608 1524 (calls to this number are charged at 7p per minute, plus your phone company's access charge)
get your Meter Point Reference Number (MPRN), which tells your energy supplier where your meter is and your gas supply number