Electricity and Gas bill checking

Since 2018 I have been convinced that our utility company have been making errors on our bills. When we provide a reading they re-bill a period of over a year even though the last reading was supplied a few months previous.

I'm looking for someone experienced in checking utility bills for accuracy, both against the prevailing rate, the meter readings, and mathemtical errors. I'm assuming that this will be either a specialist or a book keeper?