Monster bill for old property.

Hi there,

I'm not sure where else to post this, I recently received a monster bill for a property I used to share with another person that I haven't lived in for over 4 years. When I moved out I paid my share of all the bills, and removed my name from the account. When I moved to the property I'm living in currently I received the mentioned bill, which had the address of my old apartment. I rang ESB immediately to rectify the problem, and despite trying to explain the situation to the customer service woman I got no-where. Eventually she said she'd check it out and get back to me but this never happened, and I just received another bill from the ESB, this time for over €800!!! I have a two room apartment with gas heating that I'm in four days a week at the most. What am I supposed to do?