Gas company undercharged now wants payback

I bought my house from a family of 4 in July 2004. I am single and live alone. In January 2008 my gas company discovered my meter was faulty from 3/1/05 to 1/17/08 and replaced it. They now have sent me a bill for the gas not charged for due to the faulty meter. They are estimating the amount based on historical usage from prior years.
Two questions: 1) Since the meter was their responsibility and I paid my bill promptly each month, do I have legal standing to claim this is their fault and I am not responsible for the bill?
2) If I am responsible, since I as a single person consume a very different amount than a family of four, is it reasonable for me to refuse to pay until they recalculate based on a single person's historical usage?
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