My Nissan NV200 van developed a fault in August 2016 and was returned to my local dealer for repair under warranty. Since then I’ve called them numerous times and asked for a call back back with a firm date for the repair, but I’ve heard nothing definite. They gave me a small family car as a substitute, but this is totally unsuitable for my business. I have to make two or more trips to work to get everything there. The dealer keeps referring me to its customer service department who say they are waiting for a part, but that's as far as we've got in three months. What should I do?