If you're at all interested in the intricacies of gas pipes, once found, it seems that replacing the pipe comes with it's own set of difficulties. The old pipes are steel but cannot be replaced with steel. The new pipe must be plastic but only up to the property. New pipes within the property must be copper and the plastic must be joined to the copper in box at the edge of the property. Instead of digging up the entire property however, there does seem to be a solution, in that they can thread the thinner plastic pipe through the old steel pipe, using it as a sheath. There - I hope you found that as interesting as I did!
Being a gas leak, the work is being treated as an emergency and they are hoping to get it finished as soon as possible.