Wrongful demands for payment due to their admin error which they refuse to correct

I rented a flat from 2*** and was paying my electricity to Impact Meters. I moved out and told them to use my deposit to pay the outstanding balance and give me the change. First month with the new tenant they did not change the account out of my name to the new tenant despite my having informed them that I left 31 March 2018 and to make from 1 April 2018 to be in the name of the new tenant who moved in 31 March 2018. Now they’re harassing me with letters of demand and threats etc etc. when I call them I just get referred from one person to the next. I’m tired of them now.