My used Land Rover Discovery Sport was bought from main a dealer in 2018 as an ex-demonstrator. After four months the engine was replaced under warranty. After another four months during a service, the fault was found to have reoccurred. The balancing shaft had failed again - Land Rover had apparently supplied a replacement engine without the modification. They say it’s now fixed. I have rejected this car as I am not happy that the problem won’t reoccur. Am I within my rights?