As somebody who marketed Natural Gas, and we had very specific guidelines to helping folk. I would start by contacting the Energy Ombudsman as others have suggested.
http://www.energy-omb...5-contact_details.php
What is needed is to get a parallel meter in on the line to check out the accuracy or lack of it of the existing meter, It does happen from time to time. This benchmarking would allow the supplier to maker any adjustments to the sum owing based off the measurement.
I would continue to pay towards the bill based on what they have paid in the past, adjusted for price increases. The Ombudsman should advise you of the percentage to apply over your previous bills,,,,,,,,Paying something shows that you are acting in good faith, not avoiding paying anything.
Hope that this helps.