Unfair Exploitation

My vehicle was towed to Mechspec Auto Midrand on the 11th of September 2017 as an excellent referral from BMW Midrand. I was asked if the engine could be stripped for a proper diagnosis which I agreed to. I was then given a quotation for R55 000 for repairs. I was shocked at the quote and I asked if I can take my vehicle away. I was then informed that I will have to pay for the stripping of the engine, which was another shocker. Feeling manipulated I agreed to the quotation for repairs and paid a 10% deposit. I did not hear from Mechspec for a while and then in December 2017 received a call confirming that they have now identified another problem and that they will have to increase the quote. I was extremely upset and annoyed and said I am not paying more than R50 000 for the repairs. Back and forth I eventually had to pay R54 000 for the repairs. I took my car out in January 2018 after full settlement, and within a week started hearing noises from the car; the car was over heating and water was leaking from the car. Mechspec gave me 100% guarantee on their repairs based on a full diagnosis of all problems on the car. Now 5 months later after the car has been with them for 4 of those months they want to quote me further for additional repairs and problems which never existed on my vehicle. I find this absolutely pathetic and unacceptable for such a reputable company to exploit innocent, vulnerable customers. I want a full refund of my money!!!