Brand new Mercedes defective on first day and haven't seen it since - they have my car and cash

I bought a brand new C300 on Friday 29, December 2017. The next day I experienced problems with the gear box - it was the New Year's weekend so I sms'd a representative of Sandown Motors Bryanston on Monday, 1 January and took the car in as soon as the dealership opened on Tuesday, 2 January. A representative at the dealership confirmed that there was a problem with the gearbox. I havent seen the car or my money since - that was a month ago. On Thursday, 4 January I met with representatives of Sandown Motors Bryanston at the dealership - I told them I didnt want the defective car - either they replace it with a new car (which I was advised was in their ba*****t 'reserved' for me) or return my money. They asked me to write an email to the Dealer Principal, setting out my position, which I did later that day. They sent me a document to sign, authorising them to order the parts for this brand new car. I didnt sign it as I had told them I dont want the defective car. It took the Dealer Principal a week to respond - with a rambling, incoherent email. I engaged attorneys to address a letter to them - in terms of the Consumer Protection Act, they have to give me my money back for the defective car if I request it. Again, it took them a week to respond. They eventually responded to say that the faulty gear box isn't a defect . They said I had prevented them from conducting diagnostic tests on the car - which is fascinating since I havent seen the car since I returned it to them at 09h00 on Monday, 2 January. They requested a call for today, Tuesday, 30 January. They then cancelled the call they had requested, saying that they actually werent authorised to speak on the call. The lady who is so authorised apparently told them this, before going into hospital today and therefore apparently rendering everyone else ***********. I tried to phone Mercedes Benz SA, but the lady at switchboard wouldnt put me through to Andreas Engling unless I explained to her what it was I wanted. I phoned customer services and they suggested I email a complaint through, which I did last Wednesday, 24 January, but that didnt help. I am 77 years old. I paid cash for the car and traded in my Mercedes C280 - but I dont have the new car - I dont have my old car - I dont have my money. The way that the Dealer Principal and Sandown Motors Bryanston have handled this matter is an absolute disgrace. They have no regard at all for me as their customer (which I have been for more than 30 years), no regard for customer service, no regard for their brand, product or reputation, no appreciation for the distress that this has caused me.