Although there isn't an exact match for your number in the database, have you tried calling anyone of the several entries when searching under british gas in the verified database?

For instance, you could try any of the Customer Relations or Switchboard entries.  You may have to play dumb and start talking about your gas meter changing and they'll interupt you and say you've come through to wrong dept, etc.  I then would just apologise and ask if they could kindly transfer you please because you're calling from your mobile and don't have a pen/paper to hand to write down the correct number.

Update:

There are also a couple of entries in the Unverified database.  This can't be searched as easily so you have to use IE's 'Find (on this page)...' command (Ctrl-F) and again just search for british gas.

Good luck and let us know how you get on.

What is the fax number(s)?