... if they're going to "guesstimate" on an average household bill, then thats not good enough.
It depends on whether you have never had a bill, or if they have never done a reading.

If the meter is turned on and running (you can tell this by looking at it and checking whether its spinning around) then they take a reading and deduct the initial reading that they did when you had the power connected in your name and thats your bill. They shouldnt have to do an estimate at all if your meter is accessible and in fact I would insist they do a proper reading given the circumstances.