How do i get my deposit back from landlord?

when moving out of my double room in a shared house in Southampton my landlord refused to give my deposit back because i did not give him a months notice. i rang him in October and said i would be moving out midddle of October, about two weeks time. he said fine, and then said well your ret is due soon, so don't pay me for October. when i moved out he said that i should have paid him for October and the deposit was the rent for that. i thimk he is wrong and am demanding my money back, how can i get it back? Sandra Brooks.