SHEll garage damaged my engine beyond repair

My car (VW Touran TDI) was at the shell garage in Hazey view and the attendant poured petrol into the car instead of desiel of which the garage took full responsibility and tried to drain it but didn't do a good job and my engine seized when the car was driven out of the garage, the manager JOHANN Booysen promised to have my car looked at by VW and kept the car with them of which he behind my back instead took it to a friend who deals with land rovers and BMW's and ended up replacing my engine with an over used engine that had wornout parts and other parts were broken and this resulted in oil and water mixtures, injectors failing the engine even ended up falling off the mounts almost onto the road while I was driving and I was without a car for 4months due to their poor workmanship, I have since continued to request that my car be taken to VW and have been declined, instead it keeps being sent to backyard mechanics and it continues to have problems and this has been for 1 year and 1month, in this period I have at most driven my car for about 5monthsamd the other 8months it's been sent to mechanics and currently it is parked as it has no power and won't drive faster then 80km/hr, please can you assist me because I'm currently now going legal with this matter since they are now not even answeringy what's apps and my calls but I am stranded with my business suffering since I don't have a car.