Raging against Barons!!!!!

After i got my car back from Barons Bellville last Friday - it took them almost 6 weeks to repair - i found that my car was leaking oil. The seal they replaced has a factory fault and my car needs to go back for another week. I have requested a curtesy car, but was told that they do not have any AND i must cover the insurance if one should become available... HOW tf is this possible if i am not responsible for installation of a faulty part from the Barons factory???? Please advice, as i already had to pay for rental during previous 6 week repair on my car.