Since you have no written contract, you can only try and negotiate whatever you can at this point. Think about the time it will take to negotiate vs the pay for those extra 6 hours or so. There will be a point when it will not be worth it and you just need to cut your losses and move on.

My advice: cut your losses and move on asap. You have certainly learnt a lesson the hard way which is always effective. You will never (hopefully) get into this situation because you know better now. You are wiser now.