Unfair deduction of fees for electricity bill

They took my mothers electricity account and deducted it from the deposit and then after 2 months they suddenly said she was in arrears. After investigations and emails they rectified this and she had to pay it. They also send her a notice of deactivation, if she does not pay, which was unacceptable and not her fault. With the new account now in June 2022, they are asking fees for this ... I am furious as this was not her fault. She is a pensioner and not a millionaire ... really !!!!!