Wrench Monkey is trying to make a monkey of me!

I have been ripped off by Wrench Monkey in Celie Road, Retreat. Despite my complaint to them I believe they have not rectify the situation satisfactory. My complaints are: 1. WM did not quote me before they started the repair despite offering to do so when I dropped off my bike. The bill of R4183.13 was more than the bike was worth. 2. WM failed to solve the main problem that the bike was losing power when one accelerates. 3. WM charged me R595 excluding VAT to weld the exhaust. The exhaust started leaking again after having travelled less then 30Km. When I took it back they rewelded it but said they cannot guarantee that it will not leak again. Surely if you cannot fix something you should be prepared to reverse the charge. Why should I pay over R595 for a job that only lasts 5 minutes.