Ask to physically see the paper copy of the electricity bill. I strongly suspect you are paying someones elses arrears. ESB should only be costing you 50 or 60 quid a month.
We see the physical copy each month. Bills come to our address, we pin it on fridge, quarter the total and mark off when we've each paid our quarter. Then it gets filed away in drawer. We're definately only paying for us...it's just a big, cold house!