I think you need to contact Transco. Although they do not actually sell gas, I believe that they still own the majority, if not all, of the gas transport network in the UK for domestic consumers.
Their website has a list of FAQs, one of which seems to apply to your question.
Transco is the element of British Gas (as was) that is responsible for the network of mains pipes. Yellow Pages or a quick web search should find you their contact details.