Same in Belgium. We have the red Diesel for heating up your house and agricultural purposes, and the regular one which is heavily taxed. And yes in many industrial zones throughout the country the traffic department was checking vehicles. The fines for being caught using red diesel in your car were however very very hefty, so most people did not risk it.
I have the impression that most European car manufacturers used all possible tricks out of the book to make people buy diesel, and the technology was pretty advanced, much more than petrol cars where no new things were coming onto the market. The common rule was if you do less than 20,000km per year, then a petrol car would be the cheaper option. Above that, the Diesel becomes more interesting, even taking into account the yearly taxes were higher and maintenance cost was also a bit higher.