We had the same problem with N Power. Kept sending estimated bills then I checked the meter one day and they were well out. Wanted like £400 plus off me!! The annoying thing is, according to current legislation (so I read somewhere) your gas or electricity supllier is not legally obliged to read your meter more than once every 24 months which sucks big time. Some companies do it regularly some don't.
I read mine every time a bill comes now cos I trust no-one