Energy suppliers must follow a formal complaints process regulated by Ofgem. Most issues that seem impossible to resolve are resolved quickly when you use the right escalation path.
A phone call is not a formal complaint. Email your supplier’s complaints address stating clearly what happened, what you want to resolve it, and that this is a formal complaint under their complaints procedure.
If unresolved after 8 weeks, or if you receive a deadlock letter — escalate to the Energy Ombudsman. Free service. Decisions are legally binding on suppliers. Average resolution: 6–8 weeks.
Suppliers cannot bill you for energy used more than 12 months ago if the error was their fault. If you receive a large backdated bill, this rule may protect you from paying the older portion.