I think what I would do is to take my own meter readings and calculate the bill yourself. Now when you are under billed, pay the demanded amount and then deposit the difference into an interest bearing account.
At least once a year, ask the gas company to check your meter for accuracy. When they reply stating that they have checked it and found it to be OK, file the letter.
Find out what the statute of limitations is for the gas company seeking arrears billing is, and when that time period has been reached you can then withdraw the amount in the overage account and either refund or credit the homeowners with that amount including any interest.
By doing this it seems to me that you will have protected the interests of the homeowners, and afforded the gas company multiple opportunities to correct their error, so your conscious can be clear.