Why should I pay for sub-standard work?

After I placed and order with Nelius van Wyk from Stellenbosch Glass and Aluminium and didn't get any feedback from him after a month I found out that he handed the order in at Durbanville Glass and Aluminium. According to them they only received the order a week ago but I'm now not even sure what to believe from whom anymore?<br> When they came to install my new double glazed window the opening window panes were installed on the wrong side of the frame and had to go back to the workshop to be fixed. When the window was then put into the opening it was clear that the measurements were taken too small. There were gaps of at least 2cm around the window. This was then patched up with silicon and flat aluminium pieces. 3 of the brand new window panes have scratches and some of the rubbers are missing and it was just filled up with silicon. <br> I also ordered new hinges and stays for 6 other windows. Not only did they bring the wrong type, they pitched with only 3 sets. Ever since installation day I cannot get hold of Jacques at Durbanville Glass. I left at least 4 messages but have still not heard anything back. Do I really have to pay for work like this?