Hello, my name is ***** ***** I am a qualified lawyer happy to help you today.
If they do pursue you for the bill which you have already paid, make a formal complaint and if that does not resolve the matter ask them for a deadlock letter.
Once you have the deadlock letter the next step would be to escalate this to Ofgem (their regulator) - they will investigate and liaise with the energy company which would hopefully result in no further contact or threats of debt collection. They can order the energy company to make a financial award if they have acted badly.
The service is covered by the Consumer Rights Act 2015 (when you used them) so you have a right to expect a reasonable service - if you do not then the regulator will step in but it also means you can sue for any damages or losses incurred as a result of the energy company's conduct.
You can make the complaint here : http://www.ombudsman-services.org
If they do escalate and you have a debt collector visit, tell them the bill is disputed and that they should not pursue you until the outcome of your complaint has been determined. Do not let the collectors in to your property. If they do turn up and you feel compelled to pay, you can sue the energy company for your outlay at http://www.moneyclaim.gov.uk once you send them a 14 day letter to demand the money back (and 14 days has passed). You can evidence the payment from your bank so you would have evidence to use if you had to sue them. And I have no doubt that you win that claim too - it would be a small claim so no lawyer would be required.
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