Well... that should have been a welded cill! thats why either side of the joint is slightly raised, I'm guessing it was cut as the cill was slighlty the wrong angle. Best bet buy an angle , cut to the right size to sit on top BUT before fitting fill that void with loads of white silicone , so that a/ no water can get under the cill , in the property , and B/ when you fit the joiner it will push the silicone out and around the joiner making a nice tight fit . Any excess that has squirted out can be cleaned up with rag