Build a dated account timeline
List each bill, meter reading, reading type, payment and supplier contact in date order. Mark when you moved, changed supplier, reported a meter problem or first received the older charge.
Check what the supplier knew and when
Compare the information on earlier statements with the later correction. Note whether the supplier had readings, used estimates, sent bills and explained a change. These facts can affect which questions are worth asking.
Do not turn one date into an automatic outcome
Historic-charge rules can depend on the account history, communications, meter access and exceptions. The source-backed next step is to establish the facts and ask for the calculation; the checker must not promise that a charge is cancelled.
Ask for a calculation you can audit
Ask the supplier to identify the period, readings, rates, payments, corrections and any reason it says an exception applies. Keep the original bills and your chronology so you can compare the explanation with the account record.