Its their final offer...or is it? ?

I had an accident in a store due to their negligence.I filled out an accident form. They immediatley offered me a sum and admitted liability. However I dont have legal representation at the moment and they have offered me a low amount! they said its their final offer! but to be honest its terrible and I am still seeing the physio! I have written them a 4 page letter back cos their offer doesnt cover pain and suffering, physio, trips to physio, loss of studies, affect to my life (I used to dance and go to the gym). I was going to see a no win no fee lawyer but have been told they take too much from you! help! is their final offer final?