ESB Bills not ours?

Can anyone help here, in November 2010 we got an ESB overdue bill for nearly €600 - upon enquiring - it was for a premises we lived in 2007 but we moved out during the period in question.
As proof we have our first bill in our new home starting on the date of this overdue bill (we of course notified ESB of our moving of premises) - we have written many letters and made numerous phone calls about this error however we get nowhere. We have written to 3 different people in ESB (all different addresses/sections) and have written to the CER with absolutely no joy.

We closed our account with ESB over this debacle - we got a phone call from their Closed Accounts Section and explained the situation again and was told (again) it would be looked into however we have just received a threatening letter from ESB.

We are at our wits end here - what advice does any other OP have for us? Please help - we are terrified of being blacklisted wrongly. We will simply not pay for someone else's use of Electrcity.

ESB state it is our bill but how can it be when we were paying our bill at the new premises at the exact period they state we were living in the old premises (and we have our bills as proof) ????:mad: