SSE Energy company...help please

Please can anyone help me, I work part time and im a single parent to four children, a year ago I moved house and when I rang to change my gas and Electricity supply to my new house I was told they didn't supply my new property. I ended my supply on the 13th December 2013 I now have received a bill through a debt collection agency for nearly £500.
SSE apparently made up my final bill to the 13th of September by mistake instead of 13th December and as I had moved they billed the 3 month supply to "the occupier"
now the debt recovery agency has obviously worked out it was myself in the property and sent me a letter saying I owe all this money.
I am prepared to pay for the fuel that was used and ive asked for an itemised bill, but am I entitled to any discount as they made the original mistake and as I only work part time and im on my own, money is very tight. any advise would be helpful, thank you in advance