If I were in your shoes I'd get my own KWHr meter and install it in my wiring just outside their box.
Then if/when they don't match with mine reading higher by more than a miniscule delta I'd call for repairs to their equipment and a refund of the shortage based on the ratio of the readings. Then sue them for the shortfall (plus the cost of the meter and court costs) if they don't fix up and cough up.
Don't know how it works there but over here we have a "small claims" court for small value money-only suits, where lawyers are disallowed and the rules of evidence are greatly relaxed. (The accused can transfer to a full-blown court if they want but it's not usually done.)
Before doing that, check with the company (not identifying yourself or why you want one) to see if they sell refurbished, recalibrated, and (if the type supports this) resealed used meters. Some power companies do this - for appliance repairmen, landlords who want to meter their tenants, etc. Such meters tend to be really cheap. And it would be much harder for them to question the accuracy of your meter if they had sold it to you claiming it was accurate and it had their own intact seal on it. B-)
