If a supplier bills you now for energy used months ago, first ask for the dates, meter readings and calculation behind the charge. A late bill is not automatically invalid, but Great Britain has a qualified back-billing protection that may be relevant.
What the 12-month protection means
In Great Britain, suppliers are generally restricted from recovering domestic charges for energy used more than 12 months earlier when the supplier failed to bill accurately. It is not a blanket rule cancelling every charge over a year old. Exceptions can apply, including situations where a customer has obstructed accurate billing or failed to provide requested information. The detailed facts and current rules matter.
Northern Ireland has separate energy arrangements, so do not assume the Great Britain rule applies there. Ask for advice appropriate to where you live.
Gather the facts
Request an itemised explanation showing the dates covered, the readings used, whether readings were estimated or actual, and why the bill was delayed. Compare it with old bills, meter photos, reading confirmations and supplier messages. If the reading or meter identity looks wrong, provide the evidence and ask the supplier to investigate and correct the account.
Keep paying current charges you agree are due, and explain promptly if you cannot afford a payment. Do not ignore the supplier’s letters; ask it to put its position in writing while you challenge a disputed amount.
Make a complaint if needed
Complain to the supplier in writing or through its formal complaint process. Set out the issue, what you want it to do and attach relevant evidence. Keep the complaint reference. If it has not resolved the complaint after eight weeks, or it sends a deadlock letter earlier, an eligible Great Britain consumer may be able to take it to the Energy Ombudsman. Check the Ombudsman's current remit and eligibility before applying.