Help removing the euro barrel

I'm hoping that someone can help me. I tried to change the euro lock on my back door. (It's a three point locking system where you pull up the handle and turn the key to lock the door) However the screw head which is attached to the fixing hole was damaged. In my innocence, I thought that if I drilled the fixing screw that it would release the barrel. However this has not worked according to plan. The head has sheared off the fixing screw and I still can't remove the barrel. There is still a small piece of the screw in place.
Can someone please help with some constructive advise / criticism.
Many thanks in advance.