Overcharged but fault not repaired

i have an absa vehicle insurance with inovation group. My car required repair and i called in and was refereed to Contipartner/ (ex Kwikfit) in alandale drive Pietermaritzburg in April 2015. The service at this garage was pathetic, my car was not repaired. The complaint was the engine light on with loss of power. I was charged an amount of R5000. and i was still asked to pay an additional R800 to replace a sensor. What triggered the doubt was that every person that i asked about this repair said that i was overcharged to decat my car. R1300 x2. I took my car into CARTIME and they did a diagnotic and confirmed that the repair was not done correctly or not at all. I have been driving my car for past 4 months with this problem cos the garage claimed the full amount on my policy and i cannot afford the additional amount. I believe that the original fault was the O2 sensor and that the car was misdiagnosed. Now i have to pay for his mistake. This unacceptable, All i want is the service i paid for and not to be messed around. I have established contact with RMI and also with my insurance company with the intention to lay a complaint.