‘That’ is the future. Or at least, a step towards it. It’s a diesel-electric hybrid S-Class Mercedes. See, a few months ago at the Geneva Motor Show, Mercedes-Benz’s boss of large cars, Dr Uwe Ernstberger, told TopGear.com that “a complete electric [S-Class] will be done in the future”. That goal is currently limited by battery technology. Mercedes, therefore, wants to take it step by step, which is where this hybrid S comes in. Or the S300 Blue Tec Hybrid L, to give it its full name. It features the same drivetrain as the E300 Hybrid we tested a couple of years ago. Underneath there’s a 2.1-litre four-cylinder diesel engine, with a small electric motor sandwiched between engine and seven-speed gearbox, driving the rear wheels. Wait, what? There’s a four-cylinder diesel in the biggest car Mercedes-Benz makes? Before we get to the thorny issue of…