No employee contract & commision issue

I was employed for one month short of 2 years and never issued with a contract dispite asking for one several times. I had an offer letter when I started and, a year and half later, a handbook which all staff received. I have just left my job giving a weeks stat notice (as I said no contract to state otherwise). I am using acas as they have not paid commission due for sales that have completed since I left and they are claiming its company policy that once you leave you can't claim your commission so are refusing to pay. I have nothing to say I was aware or agreed to this and there is nothing to say this in my offer letter or handbook. My offer letter from when I started says I will get paid on any sale once they are paid. How do I stand as the next step is empoyment tribunal.