Mercedes Bernz Bryanston is the worst dealership

Mercedes Benz Bryanston is the pits, they have been holding onto my car since March and they are terrible communicators. I took it in Feb for coolant assessment and they told me that the thermostat needed to be replaced. Few months later i have the same problem and they telling me that several parts of the engine are damaged and the whole engine needs to be replaced. Mind you they expect me to pay for this repair because they say i drove the car for too long. They missed a repair when i first took it in and no one wants to take responsibility for their mess. PS my car is still under Warranty