You haven't mentioned how much you have been over charged nor advised if any resolution was put to you by the company so I will set out my advice below, which is generic on the steps you will need to take to try and fix this.
Step 1 - Complain to the company
Try to resolve the dispute by explaining your problem and what you want done about it. They will have an internal complaints procedure, follow it. You must make a formal complaint to them first. Make sure you keep evidence of any written communication.
Step 2 - Get a letter of deadlock
If they refuse to do what you ask to sort out the problem, ask for a "letter of deadlock" to show you've done all you can to resolve the complaint. If they fail to respond to this final letter within a reasonable period of time (say, 14 days), you can take your complaint to the Energy ombudsman.
The Energy Ombudsman is a free, independent and impartial service. They can get the supplier to correct the problem, apologise, respond to you and explain the issue, in some cases, make a financial reward.
Step 4 - Contact the ombudsman
Contact the ombudsman to submit your complaint https://www.ombudsman-services.org/
If you have questions or would like to speak to someone you can call 0330(###) ###-####or email at *****@******.***
Or you can contact them by post:
Ombudsman Services: Communications
PO Box 730
Or by phone on 0330(###) ###-####/p>
The ombudsman will look at the dispute for you. They might contact you for more information, but there isn’t a hearing, as there is with a court case. Once the ombudsman has made its decision it will write to you and the company with details of the ‘award’.
If the award is in your favour (they agree with your complaint), this will include details of what the company must do to put things right.
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