You need to find out if your deposit was protected or not. Did you receive a letter shortly after moving in?
If it was then this isn't her decision to make. You can go to the dispute service and they will decide how much she can justifiably keep. Given the time lag, and if the inventory said it was fine when you moved out, it's unlikely she'll be able to keep anything.
If it wasn't protected then depending on when you moved in you might be able to claim back for up to three times the value of the deposit.
I recommend you read this site and look through the model letters etc to help you negotiate, but it might be best to call the Shelter helpline to talk through your own circumstances.