When you get a bill that does not closely correspond with the reading on the meter, simply read the meter and send in your own proper meter reading, asking for a correct revised bill to be sent back to you.
When you get the revised bill check that the reading on the bill is the same as the one you supplied to them, that way you know you are actually paying the proper amount for the gas you have consumed, I do this and never accept over estimated bills whether they claim to be estimated or not.
keep a close check on your meter also.