Hi Patti,
I have been having exactly the same issue as you. I moved into a new house in 09/05 and am using Powergen for both Gas and Electricity. My meter readings were estimates. In December I called with my readings and they were 5000 units more than the estimates.
I had someone come round 3 times and read the meter and I did individual readings ever day for a month but heard nothing more from Powergen. Then yesterday I received another bill still with an estimated reading. This time 7000 units lower than the actual reading.
After calling last night I was put through to a "Consumption" expert who said everyting is right and I will be getting a bill for £989.47 not including VAT for the underpaid bills!!
Not good!