How can Co-op Energy’s own failures cost me £2,000?

It didn’t debit or bill my account for years, so surely back-billing rules should applyCo-op Energy was my supplier for gas and electricity from 2014 to 2016. Although I’m no longer a customer, I’ve been paying £100 a month since last May to reduce arrears of more than £2,000 caused by its failure to debit or bill my account. I now wonder whether I do, in fact, have to pay this under the back-billing rule. When I signed up to Co-op in 2014 I paid £160.84 a mon