The work has to be done by the gas distribution company for your area. They will charge for the work. (I don't have personal knowledge of their charges but, based upon previous answers here on AB, I'd guesss that the charge might be close to �500).
Use the map, here, to find out which distribution company serves your area:
http://www.nationalgrid.com/uk/Gas/Connections /domestic/
If it's National Grid, use the links to the left of the map. If it's another company use the links on the right.
Chris