HOW could it possibly be that Jaguar is able to field a diesel-engined car while still maintaining its luxe sporting heritage? Perhaps it's to do with diesels - for quite a long time now - no longer being the primitive, noisy, gutless engines they were once known to be. In fact, the only thing about today's diesels that connects with the past is that they remain much more economical than petrol-powered equivalents. Exhaust emissions, noise levels, performance - all are petrol-comparable today. The special thing about the S-Type diesel is that it's the most impressive passenger car diesel we've yet seen. Certainly they don't come any quieter, or smoother.